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Distinctive Characters

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Title: Gem-Stones and Their Distinctive Characters

Author: George Frederick Herbert Smith

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*** START OF THE PROJECT GUTENBERG EBOOK GEM-STONES
AND THEIR DISTINCTIVE CHARACTERS ***

PLATE I
Frontispiece

1. DIAMOND 3. DIAMOND
2. DIAMOND
(Crystal)

4. AQUAMARINE 5. EMERALD 6. AQUAMARINE

9. TOPAZ
7. TOPAZ

8. EMERALD
(Crystal in matrix)

12. YELLOW SAPPHIRE
10. RUBY 11. SAPPHIRE
(Oriental Topaz)
(Crystal)

13. RUBY
15. STAR-RUBY

14. SAPPHIRE
(Crystal)

GEM-STONES

GEM-STONES
AND THEIR DISTINCTIVE CHARACTERS

BY

G. F. HERBERT SMITH
M.A., D.Sc.

OF THE BRITISH MUSEUM (NATURAL HISTORY)

WITH MANY DIAGRAMS AND THIRTY-TWO PLATES
OF WHICH THREE ARE IN COLOUR

THIRD EDITION

METHUEN & CO. LTD.
36 ESSEX STREET W.C.
LONDON

First Published March 21st 1910
Second Edition June 1913
Third Edition 1919

PREFACE

I N this edition the opportunity has been taken to correct a few misprints
and mistakes that have been discovered in the first, and to alter slightly
one or two paragraphs, but otherwise no change has been made.
G. F. H. S.
Wandsworth Common, S.W.

PREFACE TO THE FIRST EDITION

I T has been my endeavour to provide in this book a concise, yet
sufficiently complete, account of the physical characters of the mineral
species which find service in jewellery, and of the methods available for
determining their principal physical constants to enable a reader, even if
previously unacquainted with the subject, to have at hand all the
information requisite for the sure identification of any cut stone which may
be met with. For several reasons I have dealt somewhat more fully with the
branches of science closely connected with the properties of crystallized
matter than has been customary hitherto in even the most comprehensive
books on precious stones. Recent years have witnessed many changes in the
jewellery world. Gem-stones are no longer entirely drawn from a few well-
marked mineral species, which are, on the whole, easily distinguishable
from one another, and it becomes increasingly difficult for even the most
experienced eye to recognize a cut stone with unerring certainty. So long as
the only confusion lay between precious stones and paste imitations an
ordinary file was the solitary piece of apparatus required by the jeweller, but
now recourse must be had to more discriminative tests, such as the
refractive index or the specific gravity, the determination of which calls for
a little knowledge and skill. Concurrently, a keener interest is being taken in
the scientific aspect of gem-stones by the public at large, who are attracted
to them mainly by æsthetic considerations.

While the treatment has been kept as simple as possible, technical
expressions, where necessary, have not been avoided, but their meanings
have been explained, and it is hoped that their use will not prove stumbling-
blocks to the novice. Unfamiliar words of this kind often give a forbidding
air to a new subject, but they are used merely to avoid circumlocution, and
not, like the incantations of a wizard, to veil the difficulties in still deeper
gloom. For actual practical work the pages on the refractometer and its use
and the method of heavy liquids for the determination of specific gravities,
and the tables of physical constants at the end of the book, with occasional
reference, in case of doubt, to the descriptions of the several species alone
are required; other methods—such as the prismatic mode of measuring
refractive indices, or the hydrostatic way of finding specific gravities—
which find a place in the ordinary curriculum of a physics course are
described in their special application to gem-stones, but they are not so
suitable for workshop practice. Since the scope of the book is confined
mainly to the stones as they appear on the market, little has been said about
their geological occurrence; the case of diamond, however, is of exceptional
interest and has been more fully treated. The weights stated for the
historical diamonds are those usually published, and are probably in many
instances far from correct, but they serve to give an idea of the sizes of the
stones; the English carat is the unit used, and the numbers must be
increased by about 2½ per cent. if the weights be expressed in metric carats.
The prices quoted for the various species must only be regarded as
approximate, since they may change from year to year, or even day to day,
according to the state of trade and the whim of fashion.
The diagram on Plate II and most of the crystal drawings were made by
me. The remaining drawings are the work of Mr. H. H. Penton. He likewise
prepared the coloured drawings of cut stones which appear on the three
coloured plates, his models, with two exceptions, being selected from the
cut specimens in the Mineral Collection of the British Museum by
permission of the Trustees. Unfortunately, the difficulties that still beset the
reproduction of pictures in colour have prevented full justice being done to
the faithfulness of his brush. I highly appreciate the interest he took in the
work, and the care and skill with which it was executed. My thanks are due
to the De Beers Consolidated Mines Co. Ltd., and to Sir Henry A. Miers,
F.R.S., Principal of the University of London, for the illustrations of the
Kimberley and Wesselton diamond mines, and of the methods and

apparatus employed in breaking up and concentrating the blue ground; to
Messrs. I. J. Asscher & Co. for the use of the photograph of the Cullinan
diamond; to Mr. J. H. Steward for the loan of the block of the refractometer;
and to Mr. H. W. Atkinson for the illustration of the diamond-sorting
machine. My colleague, Mr. W. Campbell Smith, B.A., has most kindly
read the proof-sheets, and has been of great assistance in many ways. I hope
that, thanks to his invaluable help, the errors in the book which may have
escaped notice will prove few in number and unimportant in character. To
Mr. Edward Hopkins I owe an especial debt of gratitude for his cheerful
readiness to assist me in any way in his power. He read both the manuscript
and the proof-sheets, and the information with regard to the commercial and
practical side of the subject was very largely supplied by him. He also
placed at my service a large number of photographs, some of which—for
instance, those illustrating the cutting of stones—he had specially taken for
me, and he procured for me the jewellery designs shown on Plates IV and
V.
If this book be found by those engaged in the jewellery trade helpful in
their everyday work, and if it wakens in readers generally an appreciation of
the variety of beautiful minerals suitable for gems, and an interest in the
wondrous qualities of crystallized substances, I shall be more than satisfied.
G. F. H. S.
Wandsworth Common, S.W.

CONTENTS
CHAP. PAGE
I. Introduction 1

PART I—SECTION A
THE CHARACTERS OF GEM-STONES
II. Crystalline Form 6
III. Reflection, Refraction, and Dispersion 14
IV. Measurement of Refractive Indices 21
V. Lustre and Sheen 37
VI. Double Refraction 40
VII. Absorption Effects: Colour, Dichroism, Etc. 53
VIII. Specific Gravity 63
IX. Hardness and Cleavability 78
X. Electrical Characters 82

PART I—SECTION B
THE TECHNOLOGY OF GEM-STONES
XI. Unit of Weight 84
XII. Fashioning of Gem-Stones 88
XIII. Nomenclature of Precious Stones 109
XIV. Manufactured Stones 113
XV. Imitation Stones 124

PART II—SECTION A
PRECIOUS STONES
XVI. Diamond 128
XVII. Occurrence of Diamond 137
XVIII. Historical Diamonds 157
XIX. Corundum (Sapphire, Ruby) 172

XX. Beryl (Emerald, Aquamarine, Morganite) 184

PART II—SECTION B
SEMI-PRECIOUS STONES
XXI. Topaz 197
XXII. Spinel (Balas-Ruby, Rubicelle) 203
XXIII. Garnet 207
(a) Hessonite (Grossular, Cinnamon-Stone, Hyacinth,
211
Jacinth)
(b) Pyrope (‘Cape-Ruby’) 212
(c) Rhodolite 214
(d) Almandine (Carbuncle) 214
(e) Spessartite 216
(f) Andradite (Demantoid, Topazolite, ‘Olivine’) 216
(g) Uvarovite 218
XXIV. Tourmaline (Rubellite) 219
XXV. Peridot 225
XXVI. Zircon (Jargoon, Hyacinth, Jacinth) 228
XXVII. Chrysoberyl (Chrysolite, Cat’s-Eye, Cymophane,
233
Alexandrite)
XXVIII. Quartz (Rock-Crystal, Amethyst, Citrine, Cairngorm,
238
Cat’s-Eye, Tiger’s-Eye)
XXIX. Chalcedony, Agate, Etc. 246
XXX. Opal (White Opal, Black Opal, Fire-Opal) 249
XXXI. Felspar (Moonstone, Sunstone, Labradorite, Amazon-
254
Stone)
XXXII. Turquoise, Odontolite, Variscite 257
XXXIII. Jade (Nephrite or Greenstone, Jadeite) 260
XXXIV. Spodumene (Kunzite, Hiddenite), Iolite, Benitoite 265
XXXV. Euclase, Phenakite, Beryllonite 269
XXXVI. Enstatite (‘Green Garnet’), Diopside, Kyanite,
Andalusite, Idocrase, Epidote, Sphene, Axinite, 271
Prehnite, Apatite, Dioptase
XXXVII. Cassiterite, Anatase, Pyrites, Hematite 281

XXXVIII. Obsidian, Moldavite 283

PART II—SECTION C
ORNAMENTAL STONES
XXXIX. Fluor, Lapis Lazuli, Sodalite, Violane, Rhodonite,
Azurite, Malachite, Thulite, Marble, Apophyllite,
285
Chrysocolla, Steatite or Soapstone, Meerschaum,
Serpentine

PART II—SECTION D
ORGANIC PRODUCTS
XL. Pearl, Coral, Amber 291

TABLES
I. Chemical Composition of Gem-Stones 300
II. Colour of Gem-Stones 301
III. Refractive Indices of Gem-Stones 302
IV. Colour-Dispersion of Gem-Stones 303
V. Character of the Refraction of Gem-Stones 303
VI. Dichroism of Gem-Stones 304
VII. Specific Gravities of Gem-Stones 305
VIII. Degrees of Hardness of Gem-Stones 305
IX. Data 306
Index 307

LIST OF PLATES
PAGE
I. Gem-Stones (in colour) Frontispiece
II. Refractive Index Diagram 36
III. Interference Figures 48
IV. Jewellery Designs 62
V. Jewellery Designs 88
VI. Appliances used for Polishing Diamonds 102
VII. Polishing Diamonds 103
VIII. Slitting and Polishing Coloured Stones 104
IX. Faceting Machine 105
X. Lapidary’s Workshop and Office in England 106
XI. Lapidary’s Workshop in Russia 107
XII. French Family Cutting Stones 108
XIII. Indian Lapidary 109
XIV. Blowpipe used for the Manufacture of Rubies and
118
Sapphires
XV. Kimberley Mine, 1871 140
XVI. Kimberley Mine, 1872 141
XVII. Kimberley Mine, 1874 142
XVIII. Kimberley Mine, 1881 143
XIX. Kimberley Mine at the Present Day 144
XX. Wesselton (open) Mine 145
XXI. Loading the Blue Ground on the Floors, and
146
Ploughing it over
XXII. Washing-Machines for Concentrating the Blue
147
Ground
XXIII. Diamond-Sorting Machines 148
XXIV. Kafirs Picking out Diamonds 149
XXV. Cullinan Diamond (natural size) 168

XXVI. Large Aquamarine Crystal (one-sixth natural
size), Found at Marambaya, Minas Geraes, 196
Brazil
XXVII. Gem-Stones (in colour) 226
XXVIII. Opal Mines, White Cliffs, New South Wales 252
XXIX. Gem-Stones (in colour) 256
XXX. Natives Drilling Pearls 294
XXXI. Metal Figures of Buddha Inserted in a Pearl-
296
Oyster
XXXII. Sections of Culture Pearl 297

GEM-STONES

CHAP T E R I

INTRODUCTION

B EAUTY, durability, and rarity: such are the three cardinal virtues of a
perfect gem-stone. Stones lacking any of them cannot aspire to a high
place in the ranks of precious stones, although it does not necessarily follow
that they are of no use for ornamental purposes. The case of pearl, which,
though not properly included among gem-stones, being directly produced
by living agency, yet holds an honoured place in jewellery, constitutes to
some extent an exception, since its incontestable beauty atones for its
comparative want of durability.
That a gem-stone should be a delight to the eye is a truism that need not
be laboured; for such is its whole raison d’être. The members of the
Mineral Kingdom that find service in jewellery may be divided into three
groups, according as they are transparent, translucent, or opaque. Of these
the first, which is by far the largest and the most important, may itself be
further sub-divided into two sections: stones which are devoid of colour,
and stones which are tinted. Among the former, diamond reigns supreme,
since it alone possesses that marvellous ‘fire,’ oscillating with every
movement from heavenly blue to glowing red, which is so highly esteemed
and so much besought. Other stones, such as ‘fired’ zircon, white sapphire,
white topaz, and rock-crystal, may dazzle with brilliancy of light reflected
from the surface or emitted from the interior, but none of them, like
diamond, glow with mysterious gleams. No hint of colour, save perhaps a
trace of the blue of steel, can be tolerated in stones of this category; above
all is a touch of the jaundice hue of yellow abhorred. It taxes all the skill of
the lapidary to assure that the disposition of the facets be such as to reveal
the full splendour of the stone. A coloured stone, on the other hand, depends
for its attractiveness more upon its intrinsic hue than upon the manner of its
cutting. The tint must not be too light or too dark in shade: a stone that has
barely any colour has little interest, and one which is too dark appears
almost opaque and black. The lapidary can to some extent remedy these
defects by cutting the former deep and the latter shallow. In certain curious

stones—for instance tourmaline—the transparency, and in others—such as
ruby, sapphire, and one of the recent additions to the gem world, kunzite—
the colour, varies considerably in different directions. The colours that are
most admired—the fiery red of ruby, the royal blue of sapphire, the verdant
green of emerald, and the golden yellow of topaz—are pure tints, and the
absorption spectra corresponding to them are on the whole continuous and
often restricted. They therefore retain the purity of their colour even in
artificial light, though certain sapphires transmit a relatively larger amount
of red, and consequently turn purple at night. Of the small group of
translucent stones which pass light, but are not clear enough to be seen
through, the most important is opal. It and certain others of the group owe
their merit to the same optical effect as that characterizing soap-bubbles,
tarnished steel, and so forth, and not to any intrinsic coloration. Another set
of stones—moonstone and the star-stones—reflect light from the interior
more or less regularly, but not in such a way as to produce a play of colour.
The last group, which comprises opaque stones, has a single representative
among ordinary gem-stones, namely, turquoise. In this case light is
scattered and reflected from layers immediately contiguous to the surface,
and the colour is due to the resulting absorption. The apparent darkness of a
deep-coloured stone follows from a different cause: the light passing into
the stone is wholly absorbed within it, and, since none is emitted, the stone
appears black. The claims of turquoise are maintained by the blue variety;
there is little demand for stones of a greenish tinge.
It is evidently desirable that any stones used in jewellery should be able
to resist the mechanical and chemical actions of everyday life. No one is
anxious to replace jewels every few years, and the most valuable stones are
expected to endure for all time. The mechanical abrasion is caused by the
minute grains of sand that are contained in ordinary dust, and gem-stones
should be at least as hard as they—a condition fulfilled by all the principal
species with the exception of opal, turquoise, peridot, and demantoid. Since
the beauty of the first named does not depend on the brilliancy of its polish,
scratches on the surface are not of much importance; further, all four are
only slightly softer than sand. It may be noted that the softness of paste
stones, apart from any objections that may be felt to the use of imitations,
renders them unsuitable for jewellery purposes. The only stones that are
likely to be chemically affected in the course of wear are those which are in
the slightest degree porous. It is hazardous to immerse turquoises in liquids,

even in water, lest the bluish green colour be oxidized to the despised
yellowish hue. The risk of damage to opals, moonstones, and star-stones by
the penetration of dirt or grease into the interior of the stones is less, but is
not wholly negligible. Similar remarks apply with even greater force to
pearls. Their charm, which is due to a peculiar surface-play of light, might
be destroyed by contamination with grease, ink, or similar matter; they are,
moreover, soft. For both reasons their use in rings is much to be deprecated.
Nothing can be more unsightly than the dingy appearance of a pearl ring
after a few years’ wear.
It cannot be gainsaid that mankind prefers the rare to the beautiful, and
what is within reach of all is lightly esteemed. It is for this reason that
garnet and moonstone lie under a cloud. Purchasers can readily be found for
a ‘Cape-ruby’ or an ‘olivine,’ but not for a garnet; garnets are so common, is
the usual remark. Nevertheless, the stones mentioned are really garnets. If
science succeeded in manufacturing diamonds at the cost of shillings
instead of the pounds that are now asked for Nature’s products—not that
such a prospect is at all probable or even feasible—we might expect them to
vanish entirely from fashionable jewellery.
A careful study of the showcases of the most extensive jewellery
establishment brings to light the fact that, despite the apparent profusion,
the number of different species represented is restricted. Diamond, ruby,
emerald, sapphire, pearl, opal, turquoise, topaz, amethyst are all that are
ordinarily asked for. Yet, as later pages will show, there are many others
worthy of consideration; two among them—peridot and tourmaline—are,
indeed, slowly becoming known. For the first five of the stones mentioned
above, the demand is relatively steady, and varies absolutely only with the
purchasing power of the world; but a lesser known stone may suddenly
spring into prominence owing to the caprice of fashion or the preference of
some great lady or leader of fashion. Not many years ago, for instance,
violet was the favourite colour for ladies’ dresses, and consequently
amethysts were much worn to match, but with the change of fashion they
speedily sank to their former obscurity. Another stone may perhaps figure at
some royal wedding; for a brief while it becomes the vogue, and afterwards
is seldom seen.
Except that diamond, ruby, emerald, and sapphire, and, we should add,
pearl, may indisputably be considered to occupy the first rank, it is

impossible to form the gem-stones in any strict order. Every generation sees
some change. The value of a stone is after all merely what it will fetch in
the open market, and its artistic merits may be a matter of opinion. The
familiar aphorism, de gustibus non est disputandum, is a warning not to
enlarge upon this point.

PART I—SECTION A

THE CHARACTERS OF GEM-STONES

CHAP T E R I I

CRYSTALLINE FORM

W ITH the single exception of opal, the whole of the principal mineral
species used in jewellery are distinguished from glass and similar
substances by one fundamental difference: they are crystallized matter, and
the atoms composing them are regularly arranged throughout the structure.
The words crystal and glass are employed in science in senses differing
considerably from those in popular use. The former of them is derived from
the Greek word κρύος, meaning ice, and was at one time used in that sense.
For instance, the old fourteenth-century reading of Psalm cxlvii. 17, which
appears in the authorized version as “He giveth his ice like morsels,” ran
“He sendis his kristall as morcels.” It was also applied to the beautiful,
lustrous quartz found among the eternal snows of the Alps, since, on
account of their limpidity, these stones were supposed, as Pliny tells us, to
consist of water congealed by the extreme cold of those regions; such at the
present day is the ordinary meaning of the word. But, when early
investigators discovered that a salt solution on evaporation left behind
groups of slender glistening prisms, each very similar to the rest, they
naturally—though, as we now know, wrongly—regarded them as
representing yet another form of congealed water, and applied the same
word to such substances. Subsequent research has shown that these salts, as
well as mineral substances occurring with natural faces in nature, have in
common the fundamental property of regularity of arrangement of the
constituent atoms, and science therefore defines by the word crystal a
substance in which the structure is uniform throughout, and all the similar
atoms composing it are arranged with regard to the structure in a similar
way.
The other word is yet more familiar; it denotes the transparent, lustrous,
hard, and brittle substance produced by the fusion of sand with soda or
potash or both which fills our windows and serves a variety of useful
purposes. Research has shown that glass, though apparently so uniform in
character, has in reality no regularity of molecular arrangement. It is, in

fact, a kind of mosaic of atoms, huddled together anyhow, but so irregular is
its irregularity that it simulates perfect regularity. Science uses the word
glass in this widened meaning. Two substances may, as a matter of fact,
have the same chemical composition, and one be a crystal and the other a
glass. For example, quartz, if heated to a high temperature, may be fused
and converted into a glass. The difference in the two types of structure may
be illustrated by a comparison between a regiment of soldiers drawn up on
parade and an ordinary crowd of people.
The crystalline form is a visible sign of the molecular arrangement, and is
intimately associated with the directional physical properties, such as the
optical characters, cleavage, etc. A study of it is not only of interest in itself,
but also of great importance to the lapidary who wishes to cut a stone to the
best advantage, and it is, moreover, of service in distinguishing stones when
in the rough state.

Fig. 1.—Cubo-Octahedra.

The development of natural faces on a crystal is far from being
haphazard, but is governed by the condition that the angles between similar
faces, whether on the same crystal or on different crystals, are equal,
however varying may be the shapes and the relative sizes of the faces (Fig.
1), and by the tendency of the faces bounding the crystal to fall into series
with parallel edges, such series being termed zones. Each species has a
characteristic type of crystallization, which may be referred to one of the
following six systems:—
1. Cubic.—Crystals in this system can be referred to three edges, which
are mutually at right angles, and in which the directional characters are
identical in value. These principal edges are known as axes. Some typical
forms are the cube (Fig. 2), characteristic of fluor; the octahedron (Fig. 3),
characteristic of diamond and spinel; the dodecahedron (Fig. 4),

characteristic of garnet; and the triakisoctahedron, or three-faced
octahedron (Fig. 5).

Fig. 3.— Fig. 4.—
Fig. 2.—Cube. Octahedron. Dodecahedron.

All crystals belonging to this system are singly refractive.
2. Tetragonal.—Such crystals can be referred to three axes, which are
mutually at right angles, but in only two of them are the directional
characters identical. A typical form is a four-sided prism, mm, of square
section, terminated by four triangular faces, p (Fig. 6), the usual shape of
crystals of zircon and idocrase.

Fig. 5.—
Triakisoctahedron, or
Three-faced Fig. 6.—Tetragonal
Octahedron. Crystal.

Crystals belonging to this system are doubly refractive and uniaxial, i.e.
they have one direction of single refraction (cf. p. 45), which is parallel to
the unequal edge of the three mentioned above.
3. Hexagonal.—Such crystals can be referred alternatively either to a set
of three axes, X, Y, Z (Fig. 7), which lie in a plane perpendicular to a fourth,

H, and are mutually inclined at angles of 60°, or
to a set of three, a, b, c, which are not at right
angles as in the cubic system, but in which the
directional characters are identical. The fourth
axis in the first arrangement is equally inclined to
each in the second set of axes. Many important
species crystallize in this system—corundum
(sapphire, ruby), beryl (emerald, aquamarine),
tourmaline, quartz, and phenakite. The crystals
usually display a six-sided prism, terminated by a
Fig. 7.—Two alternative sets of
Axes in the Hexagonal System.
single face, c, perpendicular to the edge of the
prism m (Fig. 8), e.g. emerald, or by six or twelve
inclined faces, p (Fig. 9), e.g. quartz, crystals of which are so constant in
form as to be the most familiar in the Mineral Kingdom. Tourmaline
crystals (Fig. 10) are peculiar because of the fact that often one end is
obviously to the eye flatter than the other.

Figs. 8–10.—Hexagonal Crystals.

Crystals belonging to this system are also doubly refractive and uniaxial,
the direction of single refraction being parallel to the fourth axis mentioned
above, and therefore also parallel to the prism edge. Hence deeply coloured
tourmaline, which strongly absorbs the ordinary ray, must be cut with the
table-facet parallel to the edge of the prism.

4. Orthorhombic.—Such crystals can be
referred to three axes, which are mutually at
right angles, but in which each of the
directional characters are different. The
crystals are usually prismatic in shape, one of
the axes being parallel to the prism edge.
Topaz, peridot, and chrysoberyl are the most
important species crystallizing in this system.
Crystals belonging to this system are
doubly refractive and biaxial, i.e. they have
Fig. 11.—Relation of the two
two directions of single refraction (cf. p. 45). directions
The three axes a, b, c (Fig. 11) are parallel of single Refraction to the Axes in
respectively to the two bisectrices of the an Orthorhombic Crystal.

directions of single refraction, and the direction perpendicular to the plane
containing those directions.
5. Monoclinic.—Such crystals can be referred to three axes, one of which
is at right angles to the other two, which are, however, themselves not at
right angles. Spodumene (kunzite) and some moonstone crystallize in this
system.
Crystals belonging to this system are doubly refractive and biaxial, but in
this case the first axis alone is parallel to one of the principal optical
directions.
6. Triclinic.—Such crystals have no edges at right angles, and the optical
characters are not immediately related to the crystalline form. Some
moonstone crystallizes in this system.
Crystals are often not single separate
individuals. For instance, diamond and spinel are
found in flat triangular crystals with their girdles
cleft at the corners (Fig. 12). Each of such
crystals is really composed of portions of two
similar octahedra, which are placed together in
such a way that each is a reflection of the other.
Fig. 12.—Twinned
Such composite crystals are called twins or
Octahedron. macles. Sometimes the twinning is repeated, and

the individuals may be so minute as to call for a microscope for their
perception.
A composite structure may also result from the conjunction of
numberless minute individuals without any definite orientation, as in the
case of chalcedony and agate. So by supposing the individuals to become
infinitesimally small, we pass to a glass-like substance.
It is often a peculiarity of crystals of a species to display a typical
combination of natural faces. Such a combination is known as the habit of
the species, and is often of service for the purpose of identifying stones
before they are cut. Thus, a habit of diamond and spinel is an octahedron,
often twinned, of garnet a dodecahedron, of emerald a flat-ended hexagonal
prism, and so on.
It is one of the most interesting and remarkable features connected with
crystallization that the composition and the physical characters—for
instance, the refractive indices and specific gravity—may, without any
serious disturbance of the molecular arrangement, vary considerably owing
to the more or less complete replacement of one element by another closely
allied to it. That is the cause of the range of the physical characters which
has been observed in such species as tourmaline, peridot, spinel, etc. The
principal replacements in the case of the gem-stones are ferric oxide, Fe2O3,
by alumina, Al2O3, and ferrous oxide, FeO, by magnesia, MgO.
A list of the principal gem-stones, arranged by their chemical
composition, is given in Table I at the end of the book.

CHAP T E R I I I

REFLECTION, REFRACTION, AND DISPERSION

I T is obvious that, since a stone suitable for ornamental use must appeal to
the eye, its most important characters are those which depend upon light;
indeed, the whole art of the lapidary consists in shaping it in such a way as
to show these qualities to the best advantage. To understand why certain
forms are given to a cut stone, it is essential for us to ascertain what
becomes of the light which falls upon the surface of the stone; further, we
shall find that the action of a stone upon light is of very great help in
distinguishing the different species of gem-stones. The phenomena
displayed by light which impinges upon the surface separating two media[1]
are very similar in character, whatever be the nature of the media.
Ordinary experience with a plane mirror tells us that, when light is
returned, or reflected, as it is usually termed, from a plane or flat surface,
there is no alteration in the size of objects viewed in this way, but that the
right and the left hands are interchanged: our right hand becomes the left
hand in our reflection in the mirror. We notice, further, that our reflection is
apparently just as far distant from the mirror on the farther side as we are on
this side. In Fig. 13 MM´ is a section of the mirror, and O´ is the image of
the hand O as seen in the mirror. Light from O reaches the eye E by way of
m, but it appears to come from O´. Since OO´ is perpendicular to the mirror,
and O and O´ lie at equal distances from it, it follows from elementary
geometry that the angle i´, which the reflected ray makes with mn, the
normal to the mirror, is equal to i, the angle which the incident ray makes
with the same direction.

Fig. 13.—Reflection at a Plane Mirror.

Again, everyday experience tells us that the case is less simple when light
actually crosses the bounding surface and passes into the other medium.
Thus, if we look down into a bath filled with water, the bottom of the bath
appears to have been raised up, and a stick plunged into the water seems to
be bent just at the surface, except in the particular case when it is perfectly
upright. Since the stick itself has not been bent, light evidently suffers some
change in direction as it passes into the water or emerges therefrom. The
passage of light from one medium to another was studied by Snell in the
seventeenth century, and he enunciated the following laws:—
1. The refracted ray lies in the plane containing the incident ray and the
normal to the plane surface separating the two media.
It will be noticed that the reflected ray obeys this law also.
2. The angle r, which the refracted ray makes with the normal, is related
to the angle i, which the incident ray makes with the same direction, by the
equation
n sin i = n´ sin r, (a)
where n and n´ are constants for the two media which are known as the
indices of refraction, or the refractive indices.

This simple trigonometrical relation may be expressed in geometrical
language. Suppose we cut a plane section through the two media at right
angles to the bounding plane, which then appears as a straight line, SOS´
(Fig. 14), and suppose that IO represents the direction of the incident ray;
then Snell’s first law tells us that the refracted ray OR will also lie in this
plane. Draw the normal NON´, and with centre O and any radius describe a
circle intersecting the incident and refracted rays in the points a and b
respectively; let drop perpendiculars ac and bd on to the normal NON´.
Then we have n.ac = n´.bd, whence we see that if n be greater than n´, ac is
less than bd, and therefore when light passes from one medium into another
which is less optically dense, in its passage across the boundary it is bent, or
refracted, away from the normal.

Fig. 14.—Refraction across a Plane Surface.

We see, then, that when light falls on the boundary of two different
media, some is reflected in the first and some is refracted into the second
medium. The relative amounts of light reflected and refracted depend on the
angle of incidence and the refractive indices of the media. We shall return to
this point when we come to consider the lustre of stones.
We will proceed to consider the course of rays at different angles of
incidence when light passes out from a medium into one less dense—for

instance, from water into air. Corresponding to light with a small angle of
incidence such as I1O (Fig. 15), some of it is reflected in the direction OI´1
and the remainder is refracted out in the direction OR1. Similarly, for the
ray I2O some is reflected along OI´2 and some refracted along OR2. Since,
in the case we have taken, the angle of refraction is greater than the angle of
incidence, the refracted ray corresponding to some incident, ray IcO will
graze the bounding surface, and corresponding to a ray beyond it, such as
I3O, which has a still greater angle of incidence, there is no refracted ray,
and all the light is wholly or totally reflected within the dense medium. The
critical angle IcON, which is called the angle of total-reflection, is very
simply related to the refractive indices of the two media; for, since r is now
a right angle, sin r = 1, and equation (a) becomes
n sin i = n´ (b)
Hence, if the angle of total-reflection is measured and one of the indices is
known, the other can easily be calculated.

Fig. 15.—Total-Reflection.

The phenomenon of total-reflection may be appreciated if we hold a glass
of water above our head, and view the light of a lamp on a table reflected
from the under surface of the water. This reflection is incomparably more
brilliant than that given by the upper surface.
The refractive index of air is taken as unity; strictly, it is that of a
vacuum, but the difference is too small to be appreciated even in very
delicate work. Every substance has different indices for light of different
colour, and it is customary to take as the standard the yellow light of a
sodium flame. This happens to be the colour to which our eyes are most
sensitive, and a flame of this kind is easily prepared by volatilizing a little
bicarbonate of soda in the flame of a bunsen burner. A survey of Table III at
the end of the book shows clearly how valuable a measurement of the
refractive index is for determining the species to which a cut stone belongs.
The values found for different specimens of the species do in cases vary
considerably owing to the great latitude possible in the chemical

constitution due to the isomorphous replacement of one element by another.
Some variation in the index may even occur in different directions within
the same stone; it results from the remarkable property of splitting up a
beam of light into two beams, which is possessed by many crystallized
substances. This forms the subject of a later chapter.
Upon the fact that the refractive index of a substance varies for light of
different colours depends such familiar phenomena as the splendour of the
rainbow and the ‘fire’ of the diamond. When white light is refracted into a
stone it no longer remains white, but is split up into a spectrum. Except in
certain anomalous substances the refractive index increases progressively as
the wave-length of the light decreases, and consequently a normal spectrum
is violet at one end and passes through green and yellow to red at the other
end. The width of the spectrum, which may be measured by the difference
between the refractive indices for the extreme red and violet rays, also
varies, though on the whole it increases with the refractive index. It is the
dispersion, as this difference is termed, that determines the ‘fire’—a
character of the utmost importance in colourless transparent stones, which,
but for it, would be lacking in interest. Diamond excels all colourless stones
in this respect, although it is closely followed by zircon, the colour of which
has been driven off by heating; it is, however, surpassed by two coloured
species: sphene, which is seldom seen in jewellery, and demantoid, the
green garnet from the Urals, which often passes under the misnomer
‘olivine.’ The dispersion of the more prominent species for the B and G
lines of the solar spectrum is given in Table IV at the end of the book.
We will now proceed to discuss methods that may be used for the
measurement of the refractive indices of cut stones.

CHAP T E R I V

MEASUREMENT OF REFRACTIVE INDICES

T HE methods available for the measurement of refractive indices are of
two kinds, and make use of two different principles. The first, which is
based upon the very simple relation found in the last chapter to subsist at
total-reflection, can be used with ease and celerity, and is best suited for
discriminative purposes; but it is restricted in its application. The second,
which depends on the measurement of the angle between two facets and the
minimum deviation experienced by a ray of light when traversing a prism
formed by them, is more involved, entails the use of more elaborate
apparatus, and takes considerable time, but it is less restricted in its
application.

(1) The Method of Total-Reflection
We see from equation b (p. 18), connecting the angle of total-reflection
with the refractive indices of the adjacent media, that, if the denser medium
be constant, the indices of all less dense media may be easily determined
from a measurement of the corresponding critical angle. In all
refractometers the constant medium is a glass with a high refractive index.
Some instruments have rotatory parts, by means of which this angle is
actually measured. Such instruments give very good results, but suffer from
the disadvantages of being neither portable nor convenient to handle, and of
not giving a result without some computation.

Fig. 16.—Refractometer (actual size).

For use in the identification of cut stones, a refractometer with a fixed
scale, such as that (Fig. 16) devised by the author, is far more convenient. In
order to facilitate the observations, a totally reflecting prism has been
inserted between the two lenses of the eyepiece. The eyepiece may be
adjusted to suit the individual eyesight; but for observers with exceptionally
long sight an adapter is provided, which permits the eyepiece being drawn
out to the requisite extent. The refractometer must be held in the manner
illustrated in Fig. 17, so that the light from a window or other source of
illumination enters the instrument by the lenticular opening underneath.
Good, even illumination of the field may also very simply be secured by
reflecting light into the instrument from a sheet of white paper laid on a
table. On looking down the eyepiece we see a scale (Fig. 18), the eyepiece
being, if necessary, focused until the divisions of the scale are clearly and
distinctly seen. Suppose, for experiment, we smear a little vaseline or
similar fatty substance on the plane surface of the dense glass, which just
projects beyond the level of the brass plate embracing it. The field of view
is now no longer uniformly illuminated, but is divided into two parts (Fig.
19): a dark portion above, which terminates in a curved edge, apparently
green in colour, and a bright portion underneath, which is composed of
totally reflected light. If we tilt the instrument downwards so that light
enters the instrument from above through the vaseline we find that the

portions of the field are reversed, the dark portion being underneath and
terminated by a red edge. It is possible so to arrange the illumination that
the two portions are evenly lighted, and the common edge becomes almost
invisible. It is therefore essential for obtaining satisfactory results that the
plate and the dense glass be shielded from the light by the disengaged hand.
The shadow-edge is curved, and is, indeed, an arc of a circle, because
spherical surfaces are used in the optical arrangements of the refractometer;
by the substitution of cylindrical surfaces it becomes straight, but sufficient
advantage is not secured thereby to compensate for the greatly increased
complexity of the construction. The shadow-edge is coloured, because the
n − nr
relative dispersion, v (nv and nr being the refractive indices for the
n
extreme violet and red rays respectively), of the vaseline differs from that of
the dense glass. The dispersion of the glass is very high, and exceeds that of
any stone for which it can be used. Certain oils have, however, nearly the
same relative dispersion, and the edges corresponding to them are
consequently almost colourless. A careful eye will perceive that the
coloured shadow-edge is in reality a spectrum, of which the violet end lies
in the dark portion of the field and the red edge merges into the bright
portion. The yellow colour of a sodium flame, which, as has already been
stated, is selected as the standard for the measurement of refractive indices,
lies between the green and the red, and the part of the spectrum to be noted
is at the bottom of the green, and practically, therefore, at the bottom of the
shadow, because the yellow and red are almost lost in the brightness of the
lower portion of the field. If a sodium flame be used as the source of
illumination, the shadow-edge becomes a sharply defined line. The scale is
so graduated and arranged that the reading where this line crosses the scale
gives the corresponding refractive index, the reading, since the line is
curved, being taken in the middle of the field on the right-hand side of the
scale. The refractometer therefore gives at once, without any intermediate
calculation, a value of the refractive index to the second place of decimals,
and a skilled observer may, by estimating the tenths of the intervals between
successive divisions, arrive at the third place; to facilitate this estimation the
semi-divisions beyond 1·650 have been inserted. The range extends nearly
to 1·800; for any substance with a higher refractive index the field is dark
as far as the limit at the bottom.

Fig. 17.—Method of Using the Refractometer.

Fig. 18.—Scale of the Fig. 19.—Shadow-edge given by a singly
Refractometer. refractive Substance.

A fat, or a liquid, wets the glass, i.e. comes into intimate contact with it,
but if a solid substance be tested in the same way, a film of air would
intervene and entirely prevent an observation. To displace it, a drop of some
liquid which is more highly refractive than the substance under test must
first be applied to the plane surface of the dense glass. The most convenient
liquid for the purpose is methylene iodide, CH2I2, which, when pure, has at
ordinary room temperatures a refractive index of 1·742. It is almost
colourless when fresh, but turns reddish brown on exposure to light. If
desired, it may be cleared in the manner described below (p. 66), but the
film of liquid actually used is so thin that this precaution is scarcely
necessary. If we test a piece of ordinary glass—one of the slips used by
microscopists for covering thin sections is very convenient for the purpose
—first applying a drop of methylene iodide to the plane surface of the dense
glass of the refractometer (Fig. 20), we notice a coloured shadow-edge
corresponding to the glass-slip at about 1·530 and another, almost
colourless, at 1·742, which corresponds to the liquid. If the solid substance
which is tested is more highly refractive than methylene iodide, only the
latter of the shadow-edges is visible, and we must utilize some more
refractive liquid. We can, however, raise the refractive index of methylene
iodide by dissolving sulphur[2] in it; the refractive index of the saturated
liquid lies well beyond 1·800 and the shadow-edge corresponding to it,
therefore, does not come within the range of the refractometer. The pure and
the saturated liquids can be procured with the instrument, the bottles
containing them being japanned on the outside to exclude light and fitted
with dipping-stoppers, by means of which a drop of the liquid required is
easily transferred to the surface of the glass of the instrument. So long as
the liquid is more highly refractive than the stone, or whatever may be the
substance under examination, its precise refractive index is of no
consequence. The facet used in the test must be flat, and must be pressed
firmly on the instrument, so that it is truly parallel to the plane surface of
the dense glass; for good results, moreover, it must be bright.

Fig. 20.—Faceted Stone in Position on the Refractometer.

We have so far assumed that the substance which we
are testing is simple and gives a single shadow-edge;
but, as may be seen from Table V, many of the gem-
stones are doubly refractive, and such will, in general,
show in the field of the refractometer two distinct
shadow-edges more or less widely separated. Suppose,
for example, we study the effect produced by a peridot,
which displays the phenomenon to a marked degree. If
we revolve the stone so that the facet under observation
remains parallel to the plane surface of the dense glass
of the refractometer and in contact with it, we notice
that both the shadow-edges in general move up or down
the scale. In particular cases, depending upon the
relation of the position of the facet selected to the
crystalline symmetry, one or both of them may remain
fixed, or one may even move across the other. But
Fig. 21.—Shadow-
whatever facet of the stone be used for the test, and edges given by a
however variable be the movements of the shadow- doubly refractive
edges, the highest and lowest readings obtainable substance.
remain the same; they are the principal indices of refraction, such as are
stated in Table III at the end of the book, and their difference measures the
maximum amount of double refraction possessed by the stone. The

procedure is therefore simplicity itself; we have merely to revolve the stone
on the instrument, usually through not more than a right angle, and note the
greatest and least readings. It will be noticed that the shadow-edges cross
the scale symmetrically in the critical and skewwise in intermediate
positions. Fig. 21 represents the effect when the facet is such as to give
simultaneously the two readings required. The shadow-edges a and b,
which are coloured in white light, correspond to the least and greatest
respectively of the principal refractive indices, while the third shadow-edge,
which is very faint, corresponds to the liquid used—methylene iodide. It is
possible, as we shall see in a later chapter, to learn from the motion, if any,
of the shadow-edges something as to the character of the double refraction.
Since, however, each shadow-edge is spectral in white light, they will not
be distinctly separate unless the double refraction exceeds the relative
dispersion. Topaz, for instance, appears in white light to yield only a single
shadow-edge, and may thus easily be distinguished from tourmaline, in
which the double refraction is large enough for the separation of the two
shadow-edges to be clearly discerned. In sodium light, however, no
difficulty is experienced in distinguishing both the shadow-edges given by
substances with small amount of double refraction, such as chrysoberyl,
quartz, and topaz, and a skilled observer may detect the separation in the
extreme instances of apatite, idocrase, and beryl. The shadow-edge
corresponding to the greater refractive index is always less distinct, because
it lies in the bright portion of the field. If the stone or its facet be small, it
must be moved on the plane surface of the dense glass until the greatest
possible distinctness is imparted to the edge or edges. If it be moved
towards the observer from the further end, a misty shadow appears to move
down the scale until the correct position is reached, when the edges spring
into view.
Any facet of a stone may be utilized so long as it is flat, but the table-
facet is the most convenient, because it is usually the largest, and it is
available even when the stone is mounted. That the stone need not be
removed from its setting is one of the great advantages of this method. The
smaller the stone the more difficult it is to manipulate; caution especially
must be exercised that it be not tilted, not only because the shadow-edge
would be shifted from its true position and an erroneous value of the
refractive index obtained, but also because a corner or edge of the stone
would inevitably scratch the glass of the instrument, which is far softer than

the hard gem-stones. Methylene iodide will in time attack and stain the
glass, and must therefore be wiped off the instrument immediately after use.

(2) The Method of Minimum Deviation
If the stone be too highly refractive for a measurement of its refractive
index to be possible with the refractometer just described, and it is desired
to determine this constant, recourse must be had to the prismatic method,
for which purpose an instrument known as a goniometer[3] is required. Two
angles must be measured; one the interior angle included between a suitable
pair of facets, and the other the minimum amount of the deviation produced
by the pair upon a beam of light traversing them.
Fig. 22 represents a section of a step-
cut stone perpendicular to a series of
facets with parallel edges; t is the table,
and a, b, c, are facets on the culet side.
The path of light traversing the prism
formed by the pair of facets, t and b, is
indicated. Suppose that A is the interior
angle of the prism, i the angle of
incidence of light at the first facet and i
´ the angle of emergence at the second
facet, and r and r´ the angles inside the
stone at the two facets respectively.
Then at the first facet light has been
bent through an angle i - r, and again at
Fig. 22.—Path at Minimum Deviation of a
the second facet through an angle i´ - r Ray
´; the angle of deviation, D, is therefore traversing a Prism formed of two Facets of
a
given by Cut Stone.
D = i + i´ - (r + r´).
We have further that
r + r´ = A,
whence it follows that
A + D = i + i´.

If the stone be mounted on the goniometer and adjusted so that the edge
of the prism is parallel to the axis of rotation of the instrument and if light
from the collimator fall upon the table-facet and the telescope be turned to
the proper position to receive the emergent beam, a spectral image of the
object-slit, or in the case of a doubly refractive stone in general, two
spectral images, will be seen in white light; in the light of a sodium flame
the images will be sharp and distinct. Suppose that we rotate the stone in the
direction of diminishing deviation and simultaneously the telescope so as to
retain an image in the field of view, we find that the image moves up to and
then away from a certain position, at which, therefore, the deviation is a
minimum. The image moves in the same direction from this position
whichever way the stone be rotated. The question then arises what are the
angles of incidence and refraction under these special conditions. It is clear
that a path of light is reversible; that is to say, if a beam of light traverses
the prism from the facet t to the facet b it can take precisely the same path
from the facet b to the facet t. Hence we should be led to expect that, since
experiment teaches us that there is only one position of minimum deviation
corresponding to the same pair of facets, the angles at the two facets must
be equal, i.e. i = i´, and r = r´. It is, indeed, not difficult to prove by either
geometrical or analytical methods that such is the case.
A A+D
Therefore at minimum deviation r = and i = and, since sin i = n
2 2
sin r, where n is the refractive index of the stone, we have the simple
relation—
A+D
sin
2
n=
A
sin
2
This relation is strictly true only when the direction of minimum
deviation is one of crystalline symmetry in the stone, and holds therefore in
general for all singly refractive stones, and for the ordinary ray of a uniaxial
stone; but the values thus obtained even in the case of biaxial stones are
approximate enough for discriminative purposes. If then the stone be singly
refractive, the result is the index required; if it be uniaxial, one value is the
ordinary index and the other image gives a value lying between the ordinary
and the extraordinary indices; if it be biaxial, the values given by the two

images may lie anywhere between the greatest and the least refractive
indices. The angle A must not be too large; otherwise the light will not
emerge at the second facet, but will be totally reflected inside the stone: on
the other hand, it must not be too small, because any error in its
determination would then seriously affect the accuracy of the value derived
for the refractive index. Although the monochromatic light of a sodium
flame is essential for precise work, a sufficiently approximate value for
discriminative purposes is obtained by noting the position of the yellow
portion of the spectral image given in white light.
In the case of a stone such as that depicted in Fig. 22 images are given by
other pairs of facets, for instance ta and tc, unless the angle included by the
former is too large. There might therefore be some doubt, to which pair
some particular image corresponded; but no confusion can arise if the
following procedure be adopted.
The table, or some easily
recognizable facet, is selected as
the facet at which light enters the
stone. The telescope is first
placed in the position in which it
is directly opposite the collimator
(T0 in Fig. 23), and clamped. The
scale is turned until it reads
exactly zero, 0° or 360°, and
clamped. The telescope is
released and revolved in the
direction of increasing readings
Fig. 23.—Course of Observations in the Method of of the scale to the position of
Minimum Deviation.
minimum deviation, T. The
reading of the scale gives at once the angle of minimum deviation, D. The
holder carrying the stone is now clamped to the scale, and the telescope is
turned to the position, T1,in which the image given by reflection from the
table facet is in the centre of the field of view; the reading of the scale is
taken. The telescope is clamped, and the scale is released and rotated until it
reads the angle already found for D. If no mistake has been made, the
reflected image from the second facet is now in the field of view. It will
probably not be quite central, as theoretically it should be, because the stone

may not have been originally quite in the position of minimum deviation, a
comparatively large rotation of the stone producing no apparent change in
the position of the refracted image at minimum deviation, and further,
because, as has already been stated, the method is not strictly true for
biaxial stones. The difference in readings, however, should not exceed 2°.
The reading, S, of the scale is now taken, and it together with 180°
subtracted from the reading for the first facet, and the value of A, the
interior angle between the two facets, obtained.
Let us take an example.
Reading T (= D) 40° 41´ Reading T1 261° 35´
less 180° 180 0
———————
81 35
Reading S 41 30
———————
½D 20 20½ A 40 5
½A 20 2½ ½A 20 2½
———————
½(A + D) 40 23
Log sin 40° 23´ 9.81151
Log sin 20 2½ 9.53492
————
Log n 0.27659
n = 1.8906.
The readings S and T are very nearly the same, and therefore we may be
sure that no mistake has been made in the selection of the facets.
In place of logarithm-tables we may make use of the diagram on Plate II.
The radial lines correspond to the angles of minimum deviation and the
skew lines to the prism angles, and the distance along the radial lines gives
the refractive index. We run our eye along the line for the observed angle of
minimum deviation and note where it meets the curve for the observed
prism angle; the refractive index corresponding to the point of intersection
is at once read off.

This method has several obvious disadvantages: it requires the use of an
expensive and elaborate instrument, an observation takes considerable time,
and the values of the principal refractive indices cannot in general be
immediately determined.
Table III at the end of the book gives the refractive indices of the gem-
stones.

PLATE II

REFRACTIVE INDEX DIAGRAM

CHAP T E R V

LUSTRE AND SHEEN

I T has been already stated that whenever light in one medium falls upon
the surface separating it from another medium some of the light is
reflected within the first, while the remainder passes out into the second
medium, except when the first is of lower refractivity than the second and
light falls at an angle greater than that of total-reflection. Similarly, when
light impinges upon a cut stone some of it is reflected and the remainder
passes into the stone. What is the relative amount of reflected light depends
upon the nature of the stone—its refractivity and hardness—and determines
its lustre; the greater the amount the more lustrous will the stone appear.
There are different kinds of lustre, and the intensity of each depends on the
polish of the surface. From a dull, i.e. an uneven, surface the reflected light
is scattered, and there are no brilliant reflections. All gem-stones take a
good polish, and have therefore, so long as the surface retains its polish,
considerable brilliancy; turquoise, on account of its softness, is always
comparatively dull.
The different kinds of lustre are—

(1) Adamantine, characteristic of diamond.
(2) Vitreous, as seen on the surface of fractured glass.
(3) Resinous, as shown by resins.

Zircon and demantoid, the green garnet called by jewellers “olivine,” alone
among gem-stones have a lustre approaching that of diamond. The
remainder all have a vitreous lustre, though varying in degree, the harder
and the more refractive species being on the whole the more lustrous.
Some stones—for instance, a cinnamon garnet—appear to have a certain
greasiness in the lustre, which is caused by stray reflections from inclusions
or other breaks in the homogeneity of the interior. A pearly lustre, which
arises from cleavage cracks and is typically displayed by the cleavage face
of topaz, would be seen in a cut stone only when flawed.

Certain corundums when viewed in the direction of the crystallographical
axis display six narrow lines of light radiating at angles of 60° from a centre
in a manner suggestive of the conventional representations of stars. Such
stones are consequently known as asterias, or more usually star-stones—
star-rubies or star-sapphires, as the case may be, and the phenomenon is
called asterism. These stones have not a homogeneous structure, but contain
tube-like cavities regularly arranged at angles of 60° in planes at right
angles to the crystallographical axis. The effect is best produced when the
stones are cut en cabochon perpendicular to that axis.
Chatoyancy is a somewhat similar phenomenon, but in this case the
fibres or cavities are parallel to a single direction, and a single broadish
band is displayed at right angles to it. Cat’s-eyes, as these stones are termed,
are cut en cabochon parallel to the fibres. The true cat’s-eye (Plate XXIX,
Fig. 1) is a variety of chrysoberyl, but the term is also often applied to
quartz showing a similar appearance. The latter is really a fibrous mineral,
such as asbestos, which has become converted into silica. The beautiful
tiger’s-eye from South Africa is a silicified crocidolite, the original blue
colour of which has been altered by oxidation to golden brown. Recently
tourmalines have been discovered which are sufficiently fibrous in structure
to display an effective chatoyancy.
The milky sheen of moonstone (Plate XXIX, Fig. 4) owes its effect to
reflections from twin lamellæ. The wonderful iridescence which is the glory
of opal, and is therefore termed opalescence, arises from a structure which
is peculiar to that species. Opal is a solidified jelly; on cooling it has
become riddled with extremely thin cracks, which were subsequently filled
with similar material of slightly different refractivity, and thus it consists of
a series of films. At the surface of each film interference of light takes place
just as at the surface of a soap-bubble, and the more evenly the films are
spaced apart the more uniform is the colour displayed, the actual tint
depending upon the thickness of the films traversed by the light giving rise
to the phenomenon.

CHAP T E R VI

DOUBLE REFRACTION

T HE optical phenomenon presented by many gem-stones is complicated
by their property of splitting up a beam of light into two with, in
general, differing characters. In this chapter we shall discuss the nature of
double refraction, as it is termed, and methods for its detection. The
phenomenon is not one that comes within the purview of everyday
experience.
So long ago as 1669 a Danish physician, by name Bartholinus, noticed
that a plate of the transparent mineral which at that time had recently been
brought over from Iceland, and was therefore called “Iceland-spar,”
possessed the remarkable property of giving a double image of objects close
to it when viewed through it. Subsequent investigation has shown that much
crystallized matter is doubly refractive, but in calcite—to use the scientific
name for the species which includes Iceland-spar—alone among common
minerals is the phenomenon so conspicuous as to be obvious to the unaided
eye. The apparent separation of the pair of images given by a plate cut or
cleaved in any direction depends upon its thickness. The large mass,
upwards of two feet (60 cm.) in thickness, which is exhibited at the far end
of the Mineral Gallery of the British Museum (Natural History), displays
the separation to a degree that is probably unique.

Fig. 24.—Apparent doubling of the Edges of a Peridot when viewed through the Table-Facet.

Although none of the gem-stones can emulate calcite in this character,
yet the double refraction of certain of them is large enough to be detected
without much difficulty. In the case of faceted stones the opposite edges
should be viewed through the table-facet, and any signs of doubling noted.
The double refraction of sphene is so large, viz. 0·08, that the doubling of
the edges is evident to the unaided eye. In peridot (Fig. 24), zircon (b), and
epidote the apparent separation of the edges is easily discerned with the
assistance of an ordinary lens. A keen eye can detect the phenomenon even
in the case of such substances as quartz with small double refraction. It
must, however, be remembered that in all such stones the refraction is
single in certain directions, and the amount of double refraction varies
therefore with the direction from nil to the maximum possessed by the
stone. Experiment with a plate of Iceland-spar shows that the rays
transmitted by it have properties differing from those of ordinary light. On
superposing a second plate we notice that there are now two pairs of
images, which are in general no longer of equal brightness, as was the case
before. If the second plate be rotated with respect to the first, two images,
one of each pair, disappear, and then the other two, the plate having turned
through a right angle between the two positions of extinction; midway
between these positions the images are all equally bright. This variation of

intensity implies that each of the rays emerging from the first plate has
acquired a one-sided character, or, as it is usually expressed, has become
plane-polarized, or, shortly, polarized.

Fig. 25.—Wave-Motion.

Before the discovery of the phenomenon of double refraction the
foundation of the modern theory of light had been laid by the genius of
Huygens. According to this theory light is the result of a wave-motion (Fig.
25) in the ether, a medium that pervades the whole of space whether
occupied by matter or not, and transmits the wave-motion at a rate varying
with the matter with which it happens to coincide. Such a medium has been
assumed because it explains satisfactorily all the phenomena of light, but it
by no means follows that it has a concrete existence. Indeed, if it has, it is
so tenuous as to be imperceptible to the most delicate experiments. The
wave-motion is similar to that observed on the surface of still water when
disturbed by a stone flung into it. The waves spread out from the source of
disturbance; but, although the waves seem to advance, the actual particles
of water merely move up and down, and have no motion at all in the
direction in which the waves are moving. If we imagine similar motion to
take place in any plane and not only the horizontal, we form some idea of
the nature of ordinary light. But after passing through a plate of Iceland-
spar, light no longer vibrates in all directions, but in each beam the
vibrations are parallel to a particular plane, the two planes being at right
angles. The exact relation of the direction of the vibrations to the plane of
polarization is uncertain, although it undoubtedly lies in the plane
containing the direction of the ray of light and the perpendicular to the
plane of polarization. The waves for different colours differ in their length,
i.e. in the distance, 2 bb (Fig. 25), from crest to crest, while the velocity,
which remains the same for the same medium, is proportional to the wave-
length. The intensity of the light varies as the square of the amplitude of the
wave, i.e. the height, ab, of the crest from the mean level.

Various methods have been proposed for obtaining polarized light. Thus
Seebeck found in 1813 that a plate of brown tourmaline cut parallel to the
crystallographic axis and of sufficient thickness (cf. p. 11) transmits only
one ray, the other being entirely absorbed within the plate. Another method
was to employ a glass plate to reflect light at a certain critical angle. The
most efficient method, and that in general use at the present day, is due to
the invention of Nicol. A rhomb of Iceland-spar (Fig. 26), of suitable length,
is sliced along the longer diagonal, dd, and the halves are cemented together
by means of canada balsam. One ray, ioo, is totally reflected at the surface
separating the mineral and the cement, and does not penetrate into the other
half; while the other ray, iee, is transmitted with almost undiminished
intensity. Such a rhomb is called a Nicol’s prism after its inventor, or
briefly, a nicol.

Fig. 26.—Nicol’s Prism.

If one nicol be placed above another and their corresponding principal
planes be at right angles no light is transmitted through the pair. In the
polarizing microscope one such nicol, called the polarizer, is placed below
the stage, and the other, called the analyser, is either inserted in the body of
the microscope or placed above the eyepiece, and the pair are usually set in
the crossed position so that the field of the microscope is dark. If a piece of
glass or a fragment of some singly refractive substance be placed on the
stage the field still remains dark; but in case of a doubly refractive stone the
field is no longer dark except in certain positions of the stone. On rotation
of the plate, or, if possible, of the nicols together, the field passes from
darkness to maximum brightness four times in a complete revolution, the
relative angular intervals between these positions being right angles. These
positions of darkness are known as the positions of extinction, and the plate
is said to extinguish in them. This test is exceedingly delicate and reveals

the double refraction even when the greatest difference in the refractive
indices is too small to be measured directly.
Doubly refractive substances are of two kinds: uniaxial, in which there is
one direction of single refraction, and biaxial, in which there are two such
directions. In the case of the former the direction of one, the ordinary ray, is
precisely the same as if the refraction were single, but the refractive index
of the other ray varies from that of the ordinary ray to a second limiting
value, the extraordinary refractive index, which may be either greater or
less. If the extraordinary is greater than the ordinary refractive index the
double refraction is said to be positive; if less, to be negative. A biaxial
substance is more complex. It possesses three principal directions, viz., the
bisectrices of the directions of single refraction and the perpendicular to the
plane containing them. The first two correspond to the greatest and least,
and the last to the mean of the principal indices of refraction. If the acute
bisectrix corresponds to the least refractive index, the double refraction is
said to be positive, and if to the greatest, negative. The relation of the
directions of single refraction, s, to the three principal directions, a, b, c, is
illustrated in Fig. 27 for the case of topaz, a positive mineral. The refractive
indices of the rays traversing one of the principal directions have the values
corresponding to the other two. In the direction a we should measure the
greatest and the mean of the principal refractive indices, in the direction b
the greatest and the least, and in the direction c the mean and the least. The
maximum amount of double refraction is therefore in the direction b.
In the examination of a faceted stone, of the most usual shape, the
simplest method is to lay the large facet, called the table, on a glass slip and
view the stone through the small parallel facet, the culet. Should the latter
not exist, it may frequently happen that owing to internal reflection no light
emerges through the steeply inclined facets. This difficulty is easily
overcome by immersing the stone in some highly refracting oil. A glass
plate held by hand over the stone with a drop of the oil between it and the
plate serves the purpose, and is perhaps a more convenient method. A stone
which does not possess a pair of parallel facets should be viewed through
any pair which are nearly parallel.
We have stated that a plate of glass has no effect on the field. Suppose,
however, it were viewed when placed between the jaws of a tightened vice
and thus thrown into a state of strain, it would then show double refraction,

the amount of which would depend on the strain.
Natural singly refractive substances frequently
show phenomena of a similar kind. Thus
diamond sometimes contains a drop of liquid
carbonic acid, and the strain is revealed by the
coloured rings surrounding the cavity which are
seen when the stone is viewed between crossed
nicols. Double refraction is also common in
diamond even when there is no included matter
Fig. 27.—Relation of the two to explain it, and is caused by the state of strain
Directions of single Refraction
to
into which the mineral is thrown on release from
the principal Optical the enormous pressure under which it was
Directions formed. Other minerals which display these so-
in a Biaxial Crystal.
called optical anomalies, such as fluor and
garnet, are not really quite singly refractive at ordinary temperatures; each
crystal is composed of several double refractive individuals. But all such
phenomena cannot be confused with the characters of minerals which
extinguish in the ordinary way, since the stone will extinguish in small
patches and these will not be dark all at the same time; further, the double
refraction is small, and on revolving the stone between crossed nicols the
extinction is not sharp. Paste stones are sometimes in a state of strain, and
display slight, but general, double refraction. Hence the existence of double
refraction does not necessarily prove that the stone is real and not an
imitation. Stones may be composed of two or more individuals which are
related to each other by twinning, in which case each individual would in
general extinguish separately. Such individuals would be larger and would
extinguish more sharply than the patches of an anomalous stone.
An examination in convergent light is sometimes of service. An auxiliary
lens is placed over the condenser so as to converge the light on to the stone.
Light now traverses the stone in different directions; the more oblique the
direction the greater the distance traversed in the stone. If it be doubly
refractive, in any given direction there will be in general two rays with
differing refractive indices and the resulting effect is akin to the well-known
phenomenon of Newton’s rings, and is an instance of what is termed
interference. It may be mentioned that the interference of light (Fig. 28)
explains such common phenomena as the colours of a soap-bubble, the hues
of tarnished steel, the tints of a layer of oil floating on water, and so on.

Light, after diverging from the stone,
comes to focus a little beneath the
plane in which the image of the
stone is formed. An auxiliary lens
must, therefore, be inserted to bring
the focal planes together, so that the
interference picture may be viewed
by means of the same eyepiece.
If a uniaxial crystal be examined
Fig. 28.—Interference of Light. along the crystallographic axis in
convergent light an interference
picture will be seen of the kind illustrated on Plate III. The arms of a black
cross meet in the centre of the field, which is surrounded by a series of
circular rings, coloured in white light. Rotation of the stone about the axis
produces no change in the picture.

PLATE III

1. UNIAXIAL 2. UNIAXIAL
(Circular Polarization)

3. BIAXIAL 4. BIAXIAL
(Crossed Brushes) (Hyperbolic Brushes)

INTERFERENCE FIGURES

A biaxial substance possesses two directions (the optic axes) along which
a single beam is transmitted. If such a stone be examined along the line
bisecting the acute angle between the optic axes (the acute bisectrix) an
interference picture[4] will be seen which in particular positions of the stone
with respect to the crossed nicols takes the forms illustrated on Plate III. As
before, there is a series of rings which are coloured in white light; they,
however, are no longer circles but consist of curves known as lemniscates,
of which the figure of 8 is a special form. Instead of an unchangeable cross

there are a pair of black “brushes” which in one position of the stone are
hyperbolæ, and in that at right angles become a cross. On rotating the stone
we find that the rings move with it and are unaltered in form, whereas the
brushes revolve about two points, called the “eyes,” where the optic axes
emerge. If the observation were made along the obtuse bisectrix the angle
between the optic axes would probably be too large for the brushes to come
into the field, and the rings might not be visible in white light, though they
would appear in monochromatic light. In the case of a substance like sphene
the figure is not so simple, because the positions of the optic axes vary
greatly for the different colours and the result is exceedingly complex; in
monochromatic light, however, the usual figure is visible.
It would probably not be possible in the case of a faceted stone to find a
pair of faces perpendicular to the required direction. Nevertheless, so long
as a portion of the figures described is in the field of view, the character of
the double refraction, whether uniaxial or biaxial, may readily be
determined.
There is yet another remarkable phenomenon which must not be passed
over. Certain substances, of which quartz is a conspicuous example and in
this respect unique among the gem-stones, possess the remarkable property
of rotating the plane of polarization of a ray of light which is transmitted
parallel to the optic axis. If a plate of quartz be cut at right angles to the axis
and placed between crossed nicols in white light, the field will be coloured,
the hue changing on rotation of one nicol with respect to the other.
Examination in monochromatic light shows that the field will become dark
after a certain rotation of the one nicol with respect to the other, the amount
of which depends on the thickness of the plate. If the plate be viewed in
convergent light, an interference picture is seen as illustrated on Plate III,
which is similar to, and yet differs in some important particulars from the
ordinary interference picture of a uniaxial stone. The cross does not
penetrate beyond the innermost ring and the centre of the field is coloured
in white light. If a stone shows such a picture, it may be safely assumed to
be quartz. It is interesting to note that minerals which possess this property
have a spiral arrangement of the constituent atoms.
It has already been remarked (p. 28) that if a faceted doubly refractive
stone be rotated with one facet always in contact with the dense glass of the
refractometer the pair of shadow-edges that are visible in the field move up

or down the scale in general from or to maximum and minimum positions.
The manner in which this movement takes place depends upon the character
of the double refraction and the position of the facet under observation with
regard to the optical symmetry of the stone. In the case of a uniaxial stone,
if the facet be perpendicular to the crystallographic axis, i.e. the direction of
single refraction, neither of the shadow-edges will move. If the facet be
parallel to that direction, one shadow-edge will move up and coincide with
the other, which remains invariable in position, and away from it to a
second critical position; the latter gives the value of the extraordinary
refractive index, and the invariable shadow-edge corresponds to the
ordinary refractive index. This phenomenon is displayed by the table-facet
of most tourmalines, because for reasons given above (p. 11) they are as a
rule cut parallel to the crystallographic axis. In the case of facets in
intermediate positions, the shadow-edge corresponding to the extraordinary
refractive index moves, but not to coincidence with the invariable shadow-
edge. The case of a biaxial stone is more complex. If the facet be
perpendicular to one of the principal directions one shadow-edge remains
invariable in position, corresponding to one of the principal refractive
indices, whilst the other moves between the critical values corresponding to
the remaining two of the principal refractive indices. In the interesting case
in which the facet is parallel to the two directions of single refraction, the
second shadow-edge moves across the one which is invariable in position.
In intermediate positions of the facet both shadow-edges move, and give
therefore critical values. Of the intermediate pair, i.e. the lower maximum
and the higher minimum, one corresponds to the mean principal refractive
index, and the other depends upon the relation of the facet to the optical
symmetry. If it is desired to distinguish between them, observations must be
made on a second facet; but for discriminative purposes such exactitude is
unnecessary, since the least and the greatest refractive indices are all that
are required.
The character of the refraction of gem-stones is given in Table V at the
end of the book.

CHAP T E R VI I

ABSORPTION EFFECTS: COLOUR, DICHROISM, ETC.

W HEN white light passes through a cut stone, colour effects result
which arise from a variety of causes. The most obvious is the
fundamental colour of the stone, which is due to its selective absorption of
the light passing through it, and would characterize it before it was cut.
Intermingled with the colour in a transparent stone is the dispersive effect
known as ‘fire,’ which has already been discussed (p. 20). In many
instances the want of homogeneity is responsible for some peculiar effects
such as opalescence, chatoyancy, and asterism. These phenomena will now
be considered in fuller detail.

Colour
All substances absorb light to some extent. If the action is slight and
affects equally the whole of the visible spectrum, the stone appears white or
colourless. Usually some portion is more strongly absorbed than the rest,
and the stone seems to be coloured. What is the precise tint depends not
only upon the portions transmitted through the stone, but also upon their
relative intensities. The eye, unlike the ear, has not the power of analysis
and it cannot of itself determine how a composite colour has been made up.
Indeed, so far as it is concerned, any colour may be exactly matched by
compounding in certain proportions three simple primary colours—red,
yellow, and violet. Alexandrite, a variety of chrysoberyl, is a curious and
instructive case. The balance in the spectrum of light transmitted through it
is such that, whereas in daylight such stones appear green, in artificial light,
especially in gas-light, they are a pronounced raspberry-red (Plate XXVII,
Figs. 11, 13). The phenomenon is intensified by the strong dichroism
characteristic of this species.
The colour is the least reliable character that may be employed for the
identification of a stone, since it varies considerably in the same species,
and often results from the admixture of some metallic oxide, which has no
essential part in the chemical composition and is present in such minute

quantities as to be almost imperceptible by analysis. Who would, for
instance, imagine from their appearance that stones so markedly diverse in
hue as ruby and sapphire were really varieties of the same species,
corundum? Again, quartz, in spite of the simplicity of its composition,
displays extreme differences of tint. Nevertheless, certain varieties do
possess a distinctive colour, emerald being the most striking example, and
in other cases the trained eye can appreciate certain characteristic subtleties
of shade. At any rate, the colour is the most obvious of the physical
characters, and serves to provide a rough division of the species, and
accordingly in Table II at the end of the book the gem-stones are arranged
by their usual tints.

Dichroism
The two rays into which a doubly refractive stone splits up a ray of light
are often differently absorbed by it, and in consequence appear on
emergence differently coloured; such stones are said to be dichroic. The
most striking instance is a deep-brown tourmaline, which, except in very
thin sections, is quite opaque to the ordinary ray. The light transmitted by a
plate cut parallel to the crystallographic axis is therefore plane-polarized;
before the invention by Nicol of the prism of Iceland-spar known by his
name this was the ordinary method of obtaining light of this character (cf. p.
43). Again, in the case of kunzite and cordierite the difference in colour is
so marked as to be obvious to the unaided eye; but where the contrast is less
pronounced we require the use of an instrument called a dichroscope, which
enables the twin colours to be seen side by side.

Fig. 29.—Dichroscope (actual size).

Fig. 29 illustrates in section the construction of a
dichroscope. The instrument consists essentially of a
rhomb of Iceland-spar, S, of such a length as to give
two contiguous images (Fig. 30) of a square hole, H,
Fig. 30.—Field of the
in one end of the tube containing it. In some
Dichroscope. instruments the terminal faces of the rhomb are
ground at right angles to its length, but usually, as in
that depicted, prisms of glass, G, are cemented on to the two ends. A cap C,
with a slightly larger hole, which is circular in shape, fits on the end of the
tube, and can be moved up and down it and revolved round it, as desired.
The stone, R, to be tested may be directly attached to it by means of some
kind of wax or cement in such a way that light which has traversed it passes
into the window, H, of the instrument; the cap at the same time permits of
the rotation of the stone about the axis of the main tube of the instrument.
The dichroscope shown in the figure has a still more convenient
arrangement: it is provided with an additional attachment, A, by means of
which the stone can be turned about an axis at right angles to the length of
the tube, and thus examined in different directions. At the other end of the
main tube is placed a lens, L, of low power for viewing the twin images: the
short tube containing it can be pushed in and out for focusing purposes.
Many makers now place the rhomb close to the lens, L, and thereby require

a much smaller piece of spar; material suitable for optical purposes is fast
growing scarce.
Suppose that a plate of tourmaline cut parallel to its crystallographic axis
is fastened to the cap and the latter rotated. We should notice, on looking
through the instrument, that in the course of a complete revolution there are
two positions, orientated at right angles to one another, in which the tints of
the two images are identical, the positions of greatest contrast of tint being
midway between. If we examine a uniaxial stone in a direction at right
angles to its optic axis we obtain the colours corresponding to the ordinary
and the extraordinary rays. In any direction less inclined to the axis we still
have the colour for the ordinary ray, but the other colour is intermediate in
tint between it and that for the extraordinary ray. The phenomenon
presented by a biaxial stone is more complex. There are three principal
colours which are visible in differing pairs in the three principal optical
directions; in other directions the tints seen are intermediate between the
principal colours. Since biaxial stones have three principal colours, they are
sometimes said to be trichroic or pleochroic, but in any single direction they
have two twin colours and show dichroism. No difference at all will be
shown in directions in which a stone is singly refractive, and it is therefore
always advisable to examine a stone in more than one direction lest the first
happens to be one of single refraction. For determinative purposes it is not
necessary to note the exact shades of tint of the twin colours, because they
vary with the inherent colour of the stone, and are therefore not constant for
the same species; we need only observe, when the stone is tested with the
dichroscope, whether there is any variation of colour, and, if so, its strength.
Dichroism is a result of double refraction, and cannot exist in a singly
refractive stone. The converse, however, is not true and it by no means
follows that, because no dichroism can be detected in a stone, it is singly
refractive. A colourless stone, for instance, cannot possibly be dichroic, and
many coloured, doubly refractive stones—for example, zircon—exhibit no
dichroism, or so little that it is imperceptible. The character is always the
better displayed, the deeper the inherent colour of the stone. The deep-green
alexandrite, for instance, is far more dichroic than the lighter coloured
varieties of chrysoberyl.
If the stone is attached to the cap of the instrument, the table should be
turned towards it so as to assure that the light passing into the instrument

has actually traversed the stone. If little light enters through the opposite
coign, a drop of oil placed thereon will overcome the difficulty (cf. p. 46). It
is also necessary, for reasons mentioned above, to examine the stone in
directions as far as possible across the girdle also. A convenient, though not
strictly accurate, method is to lay the stone with the table facet on a table
and examine the light which has entered the stone and been reflected at that
facet. The stone may easily be rotated on the table, and observations thus
made in different directions in the stone. Care must be exercised in the case
of a faceted stone not to mistake the alteration in colour due to dispersion
for a dichroic effect, and the stone must be placed close to the instrument
during an observation, because otherwise the twin rays traversing the
instrument may have taken sensibly different directions in the stone.
Dichroism is an effective test in the case of ruby; its twin colours—
purplish and yellowish red—are in marked contrast, and readily distinguish
it from other red stones. Again, one of the twin colours of sapphire is
distinctly more yellowish than the other; the blue spinel, of which a good
many have been manufactured during recent years, is singly refractive, and,
of course, shows no difference of tint in the dichroscope.
Table VI at the end of the book gives the strength of the dichroism of the
gem-stones.

Absorption Spectra
A study of the chromatic character of the light transmitted by a coloured
stone is of no little interest. As was stated above, the eye has not the power
of analysing light, and to resolve the transmitted rays into their component
parts an instrument known as a spectroscope is needed. The small ‘direct-
vision’ type has ample dispersion for this purpose. It is advantageous to
employ by preference the diffraction rather than the prism form, because in
the former the intervals in the resulting spectrum corresponding to equal
differences of wave-length are the same, whereas in the latter they diminish
as the wave-length increases and accordingly the red end of the spectrum is
relatively cramped.
The absorptive properties of all doubly refractive coloured substances
vary more or less with the direction in which light traverses them according
to the amount of dichroism that they possess, but the variation is not very

noticeable unless the stone is highly dichroic. If the light transmitted by a
deep-coloured ruby be examined with a spectroscope it will be found that
the whole of the green portion of the spectrum is obliterated (Fig. 31), while
in the case of a sapphire only a small portion of the red end of the spectrum
is absorbed. Alexandrite affords especial interest. In the spectrum of the
light transmitted by it, the violet and the yellow are more or less strongly
absorbed, depending upon the direction in which the rays have passed
through the stone (Fig. 31), and the transmitted light is mainly composed of
two portions—red and green. The apparent colour of the stone depends,
therefore, upon which of the two predominates. In daylight the resultant
colour is green flecked with red and orange, the three principal absorptive
tints (cf. p. 235), but in artificial light, which is relatively stronger in the red
portion of the spectrum, the resultant colour is a raspberry-red, and there is
less apparent difference in the absorptive tints (cf. Plate XXVII, Figs. 11,
13).

Fig. 31.—Absorption Spectra.

In all the spectra just considered, and in all like them, the portions that
are absorbed are wide, the passage from blackness to colour is gradual, and
the edges deliminating them are blurred. In the spectra of certain zircons
and in almandine garnet the absorbed portions, or bands as they are called,
are narrow, and, moreover, the transition from blackness to colour is sharp
and abrupt; such stones are therefore said to display absorption-bands.
Church in 1866 was the first to notice the bands shown by zircon (Fig. 31).
Sorby thought they portended the existence of a new element, to which he
gave the name jargonium, but subsequently discovered that they were

caused by the presence of a minute trace of uranium. A yellowish-green
zircon shows the phenomenon best, and it has all the bands shown in the
figure. The spectrum varies slightly but almost imperceptibly with the
direction in the stone. Others show the bands in the yellow and green, while
others show only those in the red, and some only one of them. The bands
are not confined to stones of any particular colour, or amount of double
refraction. Again, many zircons show no bands at all, so that their absence
by no means precludes the stone from being a zircon.
Almandine is characterized by a different spectrum (Fig. 31). The band in
the yellow is the most conspicuous, and is no doubt responsible for the
purple hue of a typical almandine. The spectrum varies in strength in
different stones. Rhodolite (p. 214), a garnet lying between almandine and
pyrope, displays the same bands, and indications of them may be detected
in the spectra of pyropes of high refraction.

PLATE IV

JEWELLERY DESIGNS

CHAP T E R VI I I

SPECIFIC GRAVITY

I T is one of our earliest experiences that different substances of the same
size have often markedly different weights; thus, there is a great
difference between wood and iron, and still greater between wood and lead.
It is usual to say that iron is heavier than wood, but the statement is
misleading, because it would be possible by selecting a large enough piece
of wood to find one at least as heavy as a particular piece of iron. We have,
in fact, to compare equal volumes of the two substances, and all ambiguity
is removed if we speak of relative density or specific gravity—the former
term being usually applied to liquids and the latter to solids—instead of
weight or heaviness. The density of water at 4° C. is taken as unity, that
being the temperature at which it is highest; at other temperatures it is
somewhat lower, as will be seen from Table IX given at the end of the book.
The direct determination of the volume of an irregular solid presents almost
insuperable difficulty; but, fortunately, for finding the specific gravity it is
quite unnecessary to know the volume, as will be shown when we proceed
to consider the methods in use.
The specific gravity of a stone is a character which is within narrow
limits constant for each species, and is therefore very useful for
discriminative purposes. It can be determined whatever be the shape of the
stone, and it is immaterial whether it be transparent or not; but, on the other
hand, the stone must be unmounted and free from the setting.
The methods for the determination of the specific gravity are of two
kinds: in the first a liquid is found of the same, or nearly the same, density
as the stone, and in the second weighings are made and the use of an
accurate balance is required.

(1) Heavy Liquids
Experiment tells us that a solid substance floats in a liquid denser than
itself, sinks in one less dense, and remains suspended at any level in one of

precisely the same density. If the stone be only slightly less dense than the
liquid, it will rise to the surface; if it be just as slightly denser, it will as
surely sink to the bottom, a physical fact which has added so much to the
difficulty and danger of submarine manœuvring. If then we can find a liquid
denser than the stone to be tested, and place the latter in it, the stone will
float on the surface. If we take a liquid which is less dense than the stone
and capable of mixing with the heavier liquid, and add it to the latter, drop
by drop, gently stirring so as to assure that the density of the combination is
uniformly the same throughout, a stage is finally reached when the stone
begins to move downwards. It has now very nearly the density of the liquid,
and, if we find by some means this density, we know simultaneously the
specific gravity of the stone.
Various devices and methods are available for ascertaining the density of
liquids—for instance, Westphal’s balance; but, apart from the
inconvenience attending such a determination, the density of all liquids is
somewhat seriously affected by changes in the temperature, and it is
therefore better to make direct comparison with fragments of substances of
known specific gravity, which are termed indicators. If of two fragments
differing slightly in specific gravity one floats on the surface of a uniform
column of liquid and the other lies at the bottom of the tube containing the
liquid, we may be certain that the density of the liquid is intermediate
between the two specific gravities. Such a precaution is necessary because,
if the liquid be a mixture of two distinct liquids, the density would tend to
increase owing to the greater volatility of the lighter of them, and in any
case the density is affected by change of temperature. The specific gravity
of stones is not much altered by variation in the temperature.
A more convenient variation of this method is to form a diffusion
column, so that the density increases progressively with the depth. If the
stone under test floats at a certain level in such a column intermediate
between two fragments of known specific gravity, its specific gravity may
be found by elementary interpolation. To form a column of this kind the
lighter liquid should be poured on to the top of the heavier. Natural
diffusion gives the most perfect column, but, being a lengthy process, it
may conveniently be quickened by gently shaking the tube, and the column
thus formed gives results sufficiently accurate for discriminative purposes.

By far the most convenient liquid for ordinary use is methylene iodide,
which has already been recommended for its high refraction. It has, when
pure, a density at ordinary room-temperatures of 3·324, and it is miscible in
all proportions with benzol, whose density is O·88, or toluol, another
hydrocarbon which is somewhat less volatile than benzol, and whose
density is about the same, namely, 0·86. When fresh, methylene iodide has
only a slight tinge of yellow, but it rapidly darkens on exposure to light
owing to the liberation of iodine which is in a colloidal form and cannot be
removed by filtration. The liquid may, however, be easily cleared by
shaking it up with any substance with which the iodine combines to form an
iodide removable by filtration. Copper filings answer the purpose well,
though rather slow in action; mercury may also be used, but is not very
satisfactory, because a small amount may be dissolved and afterwards be
precipitated on to the stone under test, carrying it down to the bottom of the
tube. Caustic potash (potassium hydroxide) is also recommended; in this
case the operation should preferably be carried out in a special apparatus
which permits the clear liquid to be drawn off underneath, because water
separates out and floats on the surface. In Fig. 32 three cut stones, a quartz
(a), a beryl (b), and a tourmaline (c) are shown floating in a diffusion
column of methylene iodide and benzol. Although the beryl is only slightly
denser than the quartz, it floats at a perceptibly lower level. These three
species are occasionally found as yellow stones of very similar tint.
Various other liquids have been used or proposed for the same purpose,
of which two may be mentioned. The first of them is a saturated solution of
potassium iodide and mercuric iodide in water, which is known after the
discoverer as Sonstadt’s solution. It is a clear mobile liquid with an amber
colour, having at 12° C. a density of 3·085; it may be mixed with water to
any extent, and is easily concentrated by heating; moreover, it is durable
and not subject to alteration of any kind; on the other hand, it is highly
poisonous and cauterizes the skin, not being checked by albumen; it also
destroys brass-ware by amalgamating the metal. The second is Klein’s
solution, a clear yellow liquid which has at 15° C. a density of 3·28. It
consists of the boro-tungstate of cadmium, of which the formula is
9WO3.B2O3.2CdO.2H2O + 16Aq, dissolved in water, with which it may be
diluted. If the salt be heated, it fuses at 75° C. in its own water of
crystallization to a yellow liquid, very mobile, with a density of 3·55.

Klein’s solution is harmless, but it cannot compare for
convenience of manipulation with methylene iodide.
The most convenient procedure is to have at hand
three glass tubes, fitted with stoppers or corks, to
contain liquids of different densities—
(a) Methylene iodide reduced to 2·7; using as
indicators orthoclase 2·55, quartz 2·66, and beryl
2·74.
(b) Methylene iodide reduced to 3·1; indicators,
beryl 2·74 and tourmaline 3·10.
(c) Methylene iodide, undiluted, 3·32.
The pure liquid in the last tube should on no
account be diluted; but the density of the other two
liquids may be varied slightly, either by adding benzol Fig. 32.—Stones of
different
in order to lower it, or by allowing benzol, which has Specific Gravities
far greater volatility than methylene iodide, to floating
evaporate, or by adding methylene iodide, in order to in a Diffusion Column of
heavy Liquid.
increase it. The density of the liquids may be
ascertained approximately from the indicators.
A glance at the table of specific gravities shows that as regards the gem-
stones methylene iodide is restricted in its application, since it can be used
to test only moonstone, quartz, beryl, tourmaline, and spodumene; opal and
turquoise, being amorphous and more or less porous, should not be
immersed in liquids, lest the appearance of the stone be irretrievably
injured. Methylene iodide readily serves to distinguish the yellow quartz
from the true topaz, with which jewellers often confuse it, the latter stone
sinking in the liquid; again aquamarine floats, but the blue topaz, which is
often very similar to it, sinks in methylene iodide.
By saturating methylene iodide with iodine and iodoform, we have a
liquid (d) of density 3·6; a fragment of topaz, 3·55, may be used to indicate
whether the liquid has the requisite density. Unfortunately this saturated
solution is so dark as to be almost opaque, and is, moreover, very viscous.
Its principal use is to distinguish diamond, 3·535, from the brilliant
colourless zircon, with which, apart from a test for hardness, it may easily

be confused. It is easy to see whether the stone floats, as it would do if a
diamond. To recover a stone which has sunk, the only course is to pour off
the liquid into another tube, because it is far too dark for the position of the
stone to be seen.
It is possible to employ a similar method for still denser stones by having
recourse to Retgers’s salt, silver-thallium nitrate. This double salt is solid at
ordinary room-temperatures, but has the remarkable property of melting at a
temperature, 75° C., which is well below the point of fusion of either of its
constituents, to a clear, mobile yellow liquid, which is miscible in any
proportion with water, and has, when pure, a density of 4·6. The salt may be
purchased, or it may be prepared by mixing 100 grams of thallium nitrate
and 64 grams of silver nitrate, or similar proportions, in a little water, and
heating the whole over a water-bath, keeping it constantly stirred with a
glass rod until it is liquefied. The two salts must be mixed in the correct
proportions, because otherwise the mixture might form other double salts,
which do not melt at so low a temperature. A glance at the table of specific
gravities shows that Retgers’s salt may be used for all the gem-stones with
the single exception of zircon (b). There are, however, some objections to
its use. It is expensive, and, unless kept constantly melted, it is not
immediately available. It darkens on exposure to strong sunlight like all
silver salts, stains the skin a peculiar shade of purple which is with
difficulty removed, and in fact only by abrasion of the skin, and, like all
thallium compounds, is highly poisonous.
It is convenient to have three tubes, fitted as before with stoppers or
corks, to contain the following liquids, when heated:—
(e) Silver-thallium nitrate, reduced to 3·5; using as indicators, peridot or
idocrase 3·40 and topaz 3·53.
(f) Silver-thallium nitrate, reduced to 4·0; indicators, topaz 3·53 and
sapphire 4·03.
(g) Silver-thallium nitrate, undiluted, 4·6.
The tubes must be heated in some form of water-bath; an ordinary glass
beaker serves the purpose satisfactorily. The pure salt should never be
diluted; but the density of the contents of tubes (e) and (f) may be varied at
will, water being added in order to lower the density, and concentration by

means of evaporation or addition of the nitrate being employed in order to
increase it. To avoid the discoloration of the skin, rubber finger-stalls may
be used, and the stones should not be handled until after they have been
washed in warm water. The staining may be minimized if the hands be well
washed in hot water before being exposed to sunlight. It is advisable to
warm the stone to be tested in a tube containing water beforehand lest the
sudden heating develop cracks. A piece of platinum, or, failing that, copper
wire is of service for removing stones from the tubes; a glass rod, spoon-
shaped at one end, does equally well. It must be noted that although
Retgers’s salt is absolutely harmless to the ordinary gem-stones—with the
exception of opal and turquoise, which, as has already been stated, being to
some extent porous, should not be immersed in liquids—it attacks certain
substances, for instance, sulphides and cannot be applied indiscriminately to
minerals.
The procedure described above is intended only as a suggestion; the
method may be varied to any extent at will, depending upon the particular
requirements. If such tests are made only occasionally, a smaller number of
tubes may be used. Thus one tube may be substituted for the two marked a
and b, the liquid contained in it being diluted as required, and a series of
indicators may be kept apart in small glass tubes. On the other hand, any
one having constantly to test stones might increase the number of tubes
with advantage, and might find it useful to have at hand fragments of all the
principal species in order to make direct comparison.

(2) Direct Weighing
The balance which is necessary in both the methods described under this
head should be capable of giving results accurate to milligrams, i.e. the
thousandth part of a gram, and consistent with that restriction the beam may
be as short as possible so as to give rapid swings and thus shorten the time
taken in the observations. A good assay balance answers the purpose
admirably. Of course, it is never necessary to wait till the balance has come
to rest. The mean of the extreme readings of the pointer attached to the
beam will give the position in which it would ultimately come to rest. Thus,
if the pointer just touches the eighth division on the right-hand side and the
second on the other, the mean position is the third division on the right-hand
side (½(8 − 2) = 3). Instead of the ordinary form of chemical balance,

Westphal’s form or Joly’s spring-balance may be employed. Weighings are
made more quickly, but are not so accurate.
In refined physical work the practice known as double-weighing is
employed to obviate any slight error there may be in the suspension of the
balance. A counterpoise which is heavier than anything to be weighed is
placed in one pan, and weighed. The counterpoise is retained in its pan
throughout the whole course of the weighings. Any substance whose weight
is to be found is placed in the other pan, and weights added till the balance
swings truly again. The difference between the two sets of weights
evidently gives the weight of the substance. Balances, however, are so
accurately constructed that for testing purposes such refined precautions are
not really necessary.
It is immaterial in what notation the weighings are made, so long as the
same is used throughout, but the metric system of weights, which is in
universal use in scientific work, should preferably be employed. Jewellers,
however, use carat weights, and a subdivision to the base 2 instead of
decimals, the fractions being ½, ¼, ⅛, 1/16, 1/32, 1/64. If these weights be
employed, it will be necessary to convert these fractions into decimals, and
write ½ = ·500, ¼ ·250, ⅛ = ·125, 1/16 = ·062, 1/32 = ·031, 1/64 = ·016.

(a) Hydrostatic Weighing
The principle of this method is very simple. The stone, the specific
gravity of which is required, is first weighed in air and then when immersed
in water. If W and W´ be these weights respectively, then W − W´ is
evidently the weight of the water displaced by the stone and having
therefore the same volume as it, and the specific gravity is therefore equal
W
to .
W − Wr
If the method of double-weighing had been adopted, the formula would
be slightly altered. Thus, suppose that c corresponds to the counterpoise, w
and w´ to the stone weighed in air and water respectively; then we have W =
c−w
c − w and W´ = c − w´, and therefore the specific gravity is equal to .
w´ − w

Fig. 33.—Hydrostatic Balance.

Some precautions are necessary in practice to assure an accurate result. A
balance intended for specific gravity work is provided with an auxiliary pan
(Fig. 33), which hangs high enough up to permit of the stone being
suspended underneath. The weight of anything used for the suspension
must, of course, be determined and subtracted from the weight found for the
stone, both when in air and when in water. A piece of fine silk is generally
used for suspending the stone in water, but it should be avoided, because
the water tends to creep up it and the error thus introduced affects the first
place of decimals in the case of a one-carat stone, the value being too high.
A piece of brass wire shaped into a cage is much to be preferred. If the same
cage be habitually used, its weight in air and when immersed in water to the
customary extent in such determinations should be found once for all.
Care must also be taken to remove all air-bubbles which cling to the
stone or the cage; their presence would tend to make the value too low. The
surface tension of water which makes it cling to the wire prevents the
balance swinging freely, and renders it difficult to obtain a weighing correct
to a milligram when the wire dips into water. This difficulty may be
overcome by substituting a liquid such as toluol, which has a much smaller
surface tension.

As has been stated above, the density of water at 4° C. is taken as unity,
and it is therefore necessary to multiply the values obtained by the density
of the liquid, whatever it be, at the temperature of the observation. In Table
IX, at the end of the book, are given the densities of water and toluol at
ordinary room-temperatures. It will be noticed that a correct reading of the
temperature is far more important in the case of toluol.
Example of a Hydrostatic Determination of Specific Gravity—
Weight of stone in air = 1·471 gram
Weight of stone in water = 1·067 „
1·471 1·471
Specific gravity = = .
1·471 − 1·067 0·404
Allowing for the density of water at the temperature of the room, which
was 16° C., the specific gravity is 3·637. Had no such allowance been
made, the result would have been four units too high in the third place of
decimals. For discriminative purposes, however, such refinement is
unnecessary.

(b) Pycnometer, or Specific Gravity Bottle
The specific gravity bottle is merely one with a fairly long neck on which
a horizontal mark has been scratched, and which is closed by a ground glass
stopper. The pycnometer is a refined variety of the specific gravity bottle. It
has two openings: the larger is intended for the insertion of the stone and
the water, and is closed by a stopper through which a thermometer passes,
while the other, which is exceedingly narrow, is closed by a stopper fitting
on the outside, and is graduated to facilitate the determination of the height
of the water in it.
The stone is weighed as in the previous method. The bottle is then
weighed, and filled with water up to the mark and weighed again. The stone
is now introduced into the bottle, and the surplus water removed with
blotting-paper or otherwise until it is at the same level as before, and the
bottle with its contents is weighed. Let W be the weight of the stone, w the
weight of the bottle, W´ the weight of the bottle and the water contained in
it, and W″ the weight of the bottle when containing the stone and the water.
Then W´ − w is the weight of the water filling the bottle up to the mark, and
W″ − w − W is the reduced weight of water after the stone has been

inserted; the difference, W + W´ − W″, is the weight of the water displaced.
W
The specific gravity is therefore . As in the previous method,
W + W´ − W″
this value must be multiplied by the density of the liquid at the temperature
of the experiment. If the method of double-weighing be adopted, the
formula will be slightly modified.

Of the above methods, that of heavy liquids, as it is usually termed, is by
far the quickest and the most convenient for stones of ordinary size, the
specific gravity of which is less than the density of pure methylene iodide,
namely, 3·324, and by its aid a value may be obtained which is accurate to
the second place of decimals, a result quite sufficient for a discriminative
test. The method is applicable no matter how small the stone may be, and,
indeed, for very small stones it is the only trustworthy method; for large
stones it is inconvenient, not only because of the large quantity of liquid
required, but also on account of the difficulty in estimating with sufficient
certainty the position of the centre of gravity of the stone. A negative
determination may be of value, especially if attention be paid to the rate at
which the stone falls through the liquid; the denser the stone the faster it
will sink, but the rate depends also upon the shape of the stone. Retgers’s
salt is less convenient because of the delay involved in warming it and of
the almost inevitable staining of the hands, but its use presents no difficulty
whatever.
Hydrostatic weighing is always available, unless the stone be very small,
but the necessary weighings occupy considerable time, and care must be
taken that no error creeps into the computation, simple though it be. Even if
everything is at hand, a determination is scarcely possible under a quarter of
an hour.
The third method, which takes even longer, is intended primarily for
powdered substances, and is not recommended for cut stones, unless there
happen to be a number of tiny ones which are known to be exactly of the
same kind.
The specific gravities of the gem-stones are given in Table VII at the end
of the book.

CHAP T E R I X

HARDNESS AND CLEAVABILITY

E VERY possessor of a diamond ring is aware that diamond easily
scratches window-glass. If other stones were tried, it would be found
that they also scratched glass, but not so readily, and, if the experiment were
extended, it would be found that topaz scratches quartz, but is scratched by
corundum, which in its turn yields to the all-powerful diamond. There is
therefore considerable variation in the capacity of precious stones to resist
abrasion, or, as it is usually termed, in their hardness. To simplify the mode
of expressing this character the mineralogist Mohs about a century ago
devised the following arbitrary scale, which is still in general use.
Mohs’s Scale of Hardness
1. Talc
2. Gypsum
3. Calcite
4. Fluor
5. Apatite
6. Orthoclase
7. Quartz
8. Topaz
9. Corundum
10. Diamond

A finger-nail scratches gypsum and softer substances. Ordinary window-
glass is slightly softer than orthoclase, and a steel knife is slightly harder; a
hardened file approaches quartz in hardness, and easily scratches glass.
By saying that a stone has hardness 7 we merely mean that it will not
scratch quartz, and quartz will not scratch it. The numbers indicate an order,
and have no quantitative significance whatever. This is an important point
about which mistakes are often made. We must not, for instance, suppose

that diamond has twice the hardness of apatite. As a matter of fact, the
interval between diamond and corundum is immensely greater than that
between the latter and talc, the softest of mineral substances. Intermediate
degrees of hardness are expressed by fractions. The number 8½ for
chrysoberyl means that it scratches topaz as easily as it itself is scratched by
corundum. Pyrope garnet is slightly harder than quartz, and its hardness is
said therefore to be 7¼.
Delicate tests show that the structure of all crystallized substances is
more or less grained, like that of wood, and the hardness for the same stone
varies in different directions. Kyanite is unique in this respect, since its
hardness ranges from 5 to 7; it can therefore be scratched by a knife in some
directions, but not in others. In most substances, however, the range is so
small as to be quite imperceptible. Slight variation is also apparent in the
hardness of different specimens of the same species. The diamonds from
Borneo and New South Wales are so distinctly harder than those from South
Africa and other localities that, when first discovered, some difficulty was
experienced in cutting them. Again, lapidaries find that while Ceylon
sapphires are harder than rubies, Kashmir sapphires are softer.
Hardness is a character of fundamental importance in a stone intended for
ornamental wear, since upon it depends the durability of the polish and
brilliancy. Ordinary dust is largely composed of grains of sand, which is
quartz in a minute form, and a gem-stone should therefore be at least as
hard as that. Paste imitations are little harder than 5, and consequently, as
experience shows, their polish does not survive a few weeks’ wear.
Hardness is, however, of little use as a discriminative test except for
distinguishing between topaz or harder stone and paste. Diamond is so
much harder than other stones that it will leave a cut in glass quite different
from the scratch of even corundum. Paste, being so soft, readily yields to
the file, and is thus easily distinguished from genuine stones. In applying
the test to a cut stone, it is best to remove it from its mount and try the
effect on the girdle, because any scratch would be concealed afterwards by
the setting. Any mark should be rubbed with the finger to assure that it is
not due to powder from the scratching agent; confusion may often be
caused in this way when the two substances are of nearly the same
hardness.

The degrees of hardness of the gem-stones are given in Table VIII at the
end of the book.

It must not be overlooked that extreme hardness is compatible with
cleavability in certain directions intimately connected with the crystalline
structure; the property, in fact, characterizes many mineral species of
different degrees of hardness. Diamond can be split in four directions
parallel to the faces of the regular octahedron, a property utilized by the
lapidary for shaping a stone previous to cutting it. Topaz cleaves with
considerable ease at right angles to the principal crystallographic axis.
Felspar has two directions of cleavage nearly at right angles to one another.
The new gem-stone, kunzite, needs cautious handling owing to the facility
with which it splits in two directions mutually inclined at about 70°.
All stones are more or less brittle, and will be fractured by a sufficiently
violent blow, but the irregular surface of a fracture cannot be mistaken for
the brilliant flat surface given by a cleavage. The cleavage is by no means
induced with equal facility in the species mentioned above. A considerable
effort is required to split diamond, but in the case of topaz or kunzite
incipient cleavage in the shape of flaws may be started if the stone be
merely dropped on to a hard floor.

CHAP T E R X

ELECTRICAL CHARACTERS

T HE definite orientation of the molecular arrangement of crystallized
substances leads in many cases to attributes which vary with the
direction and are revealed by the electrical properties. If a tourmaline
crystal be heated in a gas or alcohol flame it becomes charged with
electricity, and, since it is at the same time a bad conductor, static charges of
opposite sign appear at the two ends. Topaz shows similar characters, but in
a lesser degree. Quartz, if treated in the same way, shows charges of
opposite sign on different sides, but the phenomenon may be masked by
intimate twinning and consequent overlapping of the contrary areas. The
phenomenon may also be seen when the stones are cut. The most
convenient method for detecting the existence of the electrical charges is
that devised by Kundt. A powder consisting of a mixture of red lead and
sulphur is placed in a bellows arrangement and blown through a sieve at
one end on to the stone. Owing to the friction the particles become
electrified—red lead positively and sulphur negatively—and are attracted
by the charges of opposing sign, which will therefore be betrayed by the
colour of the dust at the corresponding spot. The powder must be kept dry;
otherwise a chemical reaction may occur leading to the formation of lead
sulphide, recognizable by its black colour. Bücker has suggested as an
alternative the use of sulphur, coloured red with carmine, the negative
element, and yellow lycopodium, the positive element.
Diamond, topaz, and tourmaline are powerful enough, when electrified
by friction with a cloth, to attract fragments of paper, the electrification
being positive. Amber develops considerable negative electricity when
treated in a similar manner.
Diamond is translucent to the Röntgen (X) rays; glass, on the other hand,
is opaque to them, and this test distinguishes brilliants from paste
imitations. Diamond also, unlike glass, phosphoresces under the influence
of radium, a property characterizing also kunzite.

It will be seen that the electrical characters, although of considerable
interest to the student, are, on account of their limited application and
difficulty of test, of little service for the discrimination of gem-stones.

PART I—SECTION B

THE TECHNOLOGY OF GEM-STONES

CHAP T E R XI

UNIT OF WEIGHT

T HE system in use for recording the weights of precious stones is
peculiar to jewellery. The unit, which is known as the carat, bears no
simple relation to any unit that has existed among European nations, and
indubitably has been introduced from the East. When man in early days
sought to record the weights of small objects, he made use of the most
convenient seeds or grains which were easily obtainable and were at the
same time nearly uniform in size. In Europe the smallest unit of weight was
the barley grain. Similarly in the East the seeds of some leguminous tree
were selected. Those of the locust-tree, Ceratonia siliqua, which is common
in the countries bordering the Mediterranean, on the average weigh so
nearly a carat that they almost certainly formed the original unit. It is,
indeed, from the Greek κεράτιον, little horn, which refers to the shape of
the pods, that the word carat is derived.
It is one of the eccentricities of the jewellery trade that precision should
not have been given to the unit of weight. Not only does it vary at most of
the trade centres in the world, but it is not even always constant at each
centre. The difference is negligible in the case of single stones of ordinary
size, but becomes a matter of serious importance when large stones, or
parcels of small stones, are bought and sold, particularly when the stones
are very costly. Attempts have been made at various times to secure a
uniform standard, but as yet with only partial success. In 1871 the carat
defined as the equivalent of 0·20500 gram was suggested at a meeting of
the principal jewellers of Paris and London, and was eventually accepted in
Paris, New York, Leipzig, and Borneo. It has, however, recently been
recognized that in view of the gradual spread of the metric system of
weights and measures the most satisfactory unit is the metric carat of one-
fifth (0·2) gram. This has now been constituted the legal carat of France and
Belgium, and no doubt other countries will follow their example. The carat
weight obtaining in London weighs about 0·20530 gram, and the
approximate equivalents in the gram at other centres are as follows:—

Florence 0·19720, Madrid 0·20539, Berlin 0·20544, Amsterdam 0·20570,
Lisbon 0·20575, Frankfort-on-Main 0·20577, Vienna 0·20613, Venice
0·20700, and Madras 0·20735. The gram itself is inconveniently large to
serve as a unit for the generality of stones met with in ordinary jewellery.
The notation for expressing the sub-multiples of the carat forms another
curious eccentricity. Fractions are used which are powers of the half: thus
the half, the half of that, i.e. the quarter, and so on down to the sixty-fourth,
and the weight of a stone is expressed by a series of fractions, e.g. 3½⅛1/64
carats. In the case of diamond a single unreduced fraction to the base 64 is
substituted in place of the series of single fractions, and the weight of a
stone is stated thus, 440/64 carats. With the introduction of the metric carat the
more convenient and rational decimal notation would, of course, be
simultaneously adopted.

Figs. 34–39.—Exact Sizes of Brilliants of various Weights.

Figs. 34–39 illustrate the exact sizes of diamonds of certain weights,
when cut as brilliants. The sizes of other stones depends upon their specific
gravity, the weight varying as the volume multiplied by the specific gravity.
Quartz, for instance, has a low specific gravity and would be perceptibly
larger, weight for weight; zircon, on the other hand, would be smaller.
It has been found more convenient to select a smaller unit in the case of
pearls, namely, the pearl-grain, four of which go to the carat.
Stencil gauges are in use for measuring approximately the weight in
carats of diamond brilliants and of pearls, which in both instances must be
unmounted. A more accurate method for determining the weight of
diamonds has been devised by Charles Moe, which is applicable to either
unmounted or mounted stones. By means of callipers, which read to three-
tenths of a millimetre, the diameter and the depth of the stone are measured,

and by reference to a table the corresponding weight is found; allowance is
made for the varying fineness of the girdle, and, in the case of large stones,
for the variation from a strictly circular section.

Since this chapter was written the movement in favour of the metric carat
has made rapid progress, and this unit will soon have been adopted as the
legal standard all over the world, even in countries, such as the British Isles
and the United States, where the metric system is not in use. The advantage
of an international unit is too obvious to need arguing.

CHAP T E R XI I

FASHIONING OF GEM-STONES

A LTHOUGH many of the gem-stones have been endowed by nature
with brilliant lustrous faces and display scintillating reflections from
their surfaces, yet their form is never such as to reveal to full perfection the
optical qualities upon which their charm depends. Moreover, the natural
faces are seldom perfect; as a rule the stones are broken either through some
convulsion of the earth’s crust or in course of extraction from the matrix in
which they have lain, or they are roughened by attrition against matter of
greater hardness, or worn by the prolonged action of water, or etched by
solvents. Beautiful octahedra of diamond or spinel have been mounted
without further embellishment, but even their appearance might have been
much improved at the lapidary’s hands.
By far the oldest of the existing styles of cutting is the rounded shape
known as cabochon, a French word derived from the Latin cabo, a head. In
the days of the Roman Empire the softer stones were often treated in this
manner; such stones were supposed to be beneficial to those suffering from
short-sightedness, the reason no doubt being that transparent stones when
cut as a double cabochon formed a convex lens. According to Pliny, Nero
had an emerald thus cut, through which he was accustomed to view the
gladiatorial shows. This style of cutting was long a favourite for coloured
stones, such as emerald, ruby, sapphire, and garnet, but has been abandoned
in modern practice except for opaque, semi-opaque, and imperfect stones.
The crimson garnet, which was at one time known by the name carbuncle,
was so systematically thus cut that the word has come to signify a red
garnet of this form. It was a popular brooch-stone with our grandmothers,
but is no longer in vogue. The East still retains a taste for stones cut in the
form of beads and drilled through the centre; the beads are threaded
together, and worn as necklaces. The native lapidaries often improve the
colour of pale emeralds by lining the hole with green paint.

PLATE V

JEWELLERY DESIGNS

The cabochon form may be of three different kinds. In the first, the
double cabochon (Fig. 40), both the upper and the under sides of the stones
are curved. The curvature, however, need not be the same in each case;
indeed, it is usually markedly different. Moonstones and starstones are

generally cut very steep above and shallow underneath.
Occasionally a ruby or a sapphire is, when cut in this
way, set with the shallow side above, because the light
that has penetrated into the stone from above is more Fig. 40.—Double
wholly reflected from a steep surface with consequent (Convex) Cabochon.
increase in the glow of colour from
the stone. Opals are always cut higher on the exposed
Fig. 41.—Simple
side, but the slope of the surface varies considerably;
Cabochon. they are generally cut steeply when required for
mounting in rings. Chrysoberyl cat’s-eyes are
invariably cut with curved bases in order to preserve the weight as great as
possible. The double cabochon form with a shallow surface underneath
merges into the second kind (Fig. 41) in which the under side is plane, the
form commonly employed for quartz cat’s-eyes, and occasionally also for
carbuncles. In this type the plane side is invariably mounted downwards. In
the third form (Fig. 42) the curvature of the under surface is reversed, and
the stone is hollowed out into a concave shape. This style is reserved for
dark stones, such as carbuncles, which, if cut at all thick, would show very
little colour. A piece of foil is often placed in the hollow in order to increase
the reflection of light, and thus to heighten the colour effect.
In early days it was supposed that the extreme
hardness of diamond precluded the possibility of
fashioning it, and up to the fifteenth century all that Fig. 42.—Double
was done was to remove the gum-like skin which (Concavo-convex)
Cabochon.
disfigured the Indian stones and to polish the natural
facets. The first notable advance was made in 1475, when Louis de
Berquem discovered, as it is said quite by accident, that two diamonds if
rubbed together ground each other. With confident courage he essayed the
new art upon three large stones entrusted to him by Charles the Bold, to the
entire satisfaction of his patron. The use of wheels or discs charged with
diamond dust soon followed, but at first the lapidaries evinced their victory
over such stubborn material by grinding diamond into divers fantastic
shapes, and failed to realize how much might be done to enhance the
intrinsic beauty of the stones by the means now at their disposal. The Indian
lapidaries arrived at the same discovery independently, and Tavernier found,
when visiting the country in 1665, a large number of diamond cutters
actively employed. If the stone were perfectly clear, they contented

themselves with polishing the natural facets; but if it contained flaws or
specks, they covered it with numerous small facets haphazardly placed. The
stone was invariably left in almost its original shape, and no effort was
made to improve the symmetry.
For a long time little further progress was made, and
even nearly a century after Berquem the only regular
patterns known to Kentmann, who wrote in 1562, were
the diamond-point and the diamond-table (Figs. 43–
44). The former consisted of the natural octahedron
facets ground to regular shape, and was long employed
for the minute stones which were set in conjunction
Fig. 43.—Table Cut
with large coloured stones in rings. The table (top view).
represented considerably greater
labour. One corner of the regular octahedron was
ground down until the artificial facet thus produced was
half the width of the stone, while the opposite corner
was slightly ground.
Fig. 44.—Table Cut
(side view). Still another century elapsed before the introduction
of the rose pattern, which comprised twenty-four
triangular facets and a flat base (Figs. 45–46), the stone being nearly
hemispherical in shape. This style is said to have been the invention of
Cardinal Mazarin, but probably he was the first to have diamonds of any
considerable size cut in this form. At the present day only tiny stones are cut
as roses.
A few more years passed away, and at length at the
close of the seventeenth century diamond came by its own
when Vincenzio Peruzzi, a Venetian, introduced the
brilliant form of cutting, and revealed for the first time its
amazing ‘fire.’ Except for minor changes this form
remains to this day the standard style for the shape of
Fig. 45.—Rose
Cut (top view).
diamond, and the word brilliant is commonly employed to
denote diamond cut in this way. So obviously and
markedly superior is the style to all others that upon its discovery the
owners of large roses had them re-cut as brilliants despite the loss in weight
necessitated by the change.

The brilliant form is derived from the old table by
increasing the number of facets and slightly altering the
angles pertaining to the natural octahedron. In a perfect
brilliant (Figs. 47–49) there are altogether 58 facets, 33 Fig. 46.—Rose
above and 25 below the girdle, as the edge separating the Cut (side view).

upper and lower portions of the stone is termed, which are arranged in the
following manner. Eight star-facets, triangular in shape, immediately
surround the large table-facet. Next come four large templets or bezels,
quadrilateral in form, arranged in pairs on opposite sides of the table-facet,
the four quoins or lozenges, similar in shape, coming intermediately
between them; in modern practice, however, these two sets are identical in
shape and size, and there are consequently eight facets of the same kind
instead of two sets of four. The eight cross or skew facets and the eight skill
facets, in both sets the shape being triangular, form the boundary of the
girdle; modern brilliants usually have instead sixteen facets of the same
shape and size. The above 33 facets lie above the girdle and form the crown
of the stone. Immediately opposite and parallel to the table is the tiny culet.
Next to the latter come the four large pavilion facets with the four quoins
intermediately between them, both sets being five-sided but nearly
quadrilateral in shape; these again are usually combined into eight facets of
the same size. Eight cross facets and eight skill facets, both sets, like those
in the crown, being triangular in shape, form the lower side of the girdle;
these also are generally united into a set of sixteen similar facets. These 25
facets which lie below the girdle comprise the ‘pavilion,’ or base of the
stone. In a regular stone properly cut a templet is nearly parallel to a
pavilion, and an upper to a lower cross facet. The contour of the girdle is
usually circular, but occasionally assumes less symmetrical shapes, as for
instance in drop-stones or pendeloques, and the facets are at the same time
distorted. The number of facets may with advantage be increased in the case
of large stones. An additional set of eight star facets is often placed round
the culet, the total number then being 66. It may be mentioned that the
largest stone cut from the Cullinan has the exceptional number of 74 facets.

Fig. 47.—Brilliant Cut Fig. 48.—Brilliant Cut
(top view). (base view).

In order to secure the finest optical effect certain
proportions have been found necessary. The depth of
the crown must be one-half that of the base, and
therefore one-third the total depth of the stone, and the
width of the table must be slightly less than half that Fig. 49.—Brilliant
of the stone. The culet should be quite small, not more Cut (side view).
in width than one-sixth of the table; it is, in fact, not
required at all except to avoid the danger of the point splintering. The girdle
should be as thin as is compatible with strength sufficient to prevent
chipping in the process of mounting the stone; if it were left thick, the rough
edge would be visible by reflection at the lower facets, and would,
especially if at all dirty, seriously affect the quality of the stone. The shape
of the stone is largely determined by the sizes of the templets in the crown
and the pavilions in the base as compared with that of the table, or, what
comes to the same thing, by the inclinations at which they are cut to that
facet. If the table had actually half the width of the stone, the angle[5]
between it and a templet would be exactly half a right angle or 45°; it is,
however, made somewhat smaller, namely, about 40°. A pavilion, being
parallel to a templet, makes a similar angle with the culet. The cross facets
are more steeply inclined, and make an angle of about 45° with the table or
the culet, as the case may be. The star facets, on the other hand, slant
perceptibly less, and make an angle of only about 26° with the table. A
latitude of some 4° or 5° is possible without seriously affecting the ‘fire’ of
the stone.
The object of the disposition of the facets on a brilliant is to assure that
all the light that enters the stone, principally by way of the table, is wholly
reflected from the base and emerges through the crown, preferably by way

of the inclined facets. A brilliant-cut diamond, if viewed with the table
between the observer and the light, appears quite dark except for the small
amount of light escaping through the culet. Light should therefore fall on
the lower facets at angles greater than the critical angle of total-reflection,
which for diamond is 24° 26´. The pavilions should be inclined properly at
double this angle, or 48° 52´, to the culet; but a ray that emerges at a
pavilion in the actual arrangement entered the table at nearly grazing
incidence, and the amount of light entering this facet at such acute
perspective is negligible. On the other hand, after reflection at the base light
must, in order to emerge, fall on the crown at less than the critical angle of
total-reflection. In Fig. 50 are shown diagrammatically the paths of rays that
entered the table in divers ways. The ray emerging again at the table suffers
little or no dispersion and is almost white, but those coming out through the
inclined facets are split up into the rainbow effect, known as ‘fire,’ for
which diamond is so famous. It is in order that so much of the light entering
by the table may emerge through the inclined facets of the crown that the
pavilions are inclined at not much more than 40° to the culet. It might be
suggested that instead of being faceted the stone should be conically
shaped, truncated above and nearly complete below. The result would no
doubt be steadier, but, on the other hand, far less pleasing. It is the ever-
changing nuance that chiefly attracts the eye; now a brilliant flash of purest
white, anon a gleam of cerulean blue, waxing to richest orange and dying in
a crimson glow, all intermingled with the manifold glitter from the surface
of the stone. Absolute cleanliness is essential if the full beauty of any stone
is to be realized, but this is particularly true of diamond. If the back of the
stone be clogged with grease and dirt, as so often happens in claw-set rings,
light is no longer wholly reflected from the base; much of it escapes, and
the amount of ‘fire’ is seriously diminished.

Fig. 50.—Course of the Rays of Light passing through a Brilliant.

Needless to state, lapidaries make no careful angular measurements when
cutting stones, but judge of the position of the facets entirely by eye. It
sometimes therefore happens that the permissible limits are overstepped, in
which event the stone is dead and may resist all efforts to vivify it short of
the heroic course of re-cutting it, too expensive a treatment in the case of
small stones.
The factors that govern the properties of a brilliant-cut stone are large
colour-dispersion, high refraction, and freedom from any trace of intrinsic
colour. The only gem-stone that can vie with diamond in these respects is
zircon. Although it is rare to find a zircon naturally without colour, yet
many kinds are easily deprived of their tint by the application of heat. A
brilliant-cut zircon is, indeed, far from readily distinguished by eye from
diamond, and has probably often passed as one, but it may easily be
identified by its large double refraction (cf. p. 41) and inferior hardness.
The remaining colourless stones, such as white sapphire, topaz, and quartz
(rock-crystal), have insufficient refractivity to give total-reflection at the
base, and, moreover, they are comparatively deficient in ‘fire.’

A popular style of cutting which is much in
vogue for coloured stones is the step- or trap-cut,
consisting of a table and a series of facets with
parallel horizontal edges (Figs. 51–52) above and
Fig. 51.—Step- or Trap-Cut below the girdle; in recent jewellery, however,
(top view). the top of the stone is
often brilliant-cut. The contour may be oblong,
square, lozenge, or heart-shaped, or have less
regular forms. The table is sometimes slightly Fig. 52.—Step- or Trap-Cut
rounded. Since the object of this style is (side view).
primarily to display the intrinsic colour of the
stone and not so much a brilliant play of light from the interior, no attempt
is made to secure total-reflection at the lower facets. The stone therefore
varies in depth according to its tint; if dark, it is cut shallow, lest light be
wholly absorbed within, and the stone appear practically opaque, but if
light, it is cut deep, in order to secure fullness of tint. Much precision in
shape and disposition of the facets is not demanded, and the stones are
usually cut in such a way that, provided the desired effect is obtained, the
weight is kept as great as possible; we may recall that stones are sold by
weight. In considering what will be the optical effect of any particular
shape, regard must be had to the effective colour of the transmitted light.
For instance, although sapphire and ruby belong to the same species and
have the same refractive indices, yet, since the former transmits mainly blue
and the latter red light, they have for practical purposes appreciably
different indices, and lapidaries find it therefore possible to cut the base of
ruby thicker than that of sapphire, and thus keep the weight greater. It is
instructive too what can be done with the most unpromising material by the
exercise of a little ingenuity. Thus Ceylon sapphires are often so irregularly
coloured that considerable skill is called for in cutting them. A stone may,
for instance, be almost colourless except for a single spot of blue; yet, if the
stone be cut steeply and the spot be brought to the base, the effect will be
precisely the same as if the stone were uniformly coloured, because all the
light emerging from the stone has passed through the spot at the base and
therefore been tinted blue.
The mechanism employed in the fashioning of gem-stones is simple in
character, and comprises merely metal plates or wheels for slitting, and
discs or laps for grinding and polishing the stones, the former being set

vertically and rotated about horizontal spindles, and the latter set
horizontally and rotated about vertical spindles. Mechanical power is
occasionally used for driving both kinds of apparatus, but generally,
especially in slitting and in delicate work, hand-power is preferred. In the
East native lapidaries make use of vertical wheels (Plate XIII) also for
grinding and polishing stones, which explains why native-cut stones never
have truly plane facets; it will be noticed from the picture that a long bow is
used to drive the spindle.
Owing to the unique hardness of diamond it can be fashioned only by the
aid of its own powder. The process differs therefore materially from the
cutting of the remaining gem-stones, and will be described separately.
Indeed, so different are the two classes of work that firms seldom habitually
undertake both.
The discovery of the excellent cleavage of diamond enormously reduced
the labour of cutting large stones. A stone containing a bad flaw may be
split to convenient shape in as many minutes as the days or even weeks
required to grind it down. The improvement in the appliances and the
provision of ample mechanical power has further accelerated the process
and reduced the cost. Two years were occupied in cutting the diamond
known as the Pitt or Regent, whereas in only six months the colossal
Cullinan was shaped into two large and over a hundred smaller stones with
far less loss of material.
Although the brilliant form was derived from the regular octahedron, it
by no means follows that, because diamond can be cleaved to the latter
form, such is the initial step in fashioning the rough mass. The aim of the
lapidary is to cut the largest possible stone from the given piece of rough,
and the finished brilliant usually bears no relation whatever to the natural
octahedron. The cleavage is utilized only to free the rough of an awkward
and useless excrescence, or of flaws. Although the octahedron is one of the
common forms in which diamond is found, it is rarely regular, and oftener
than not one of the larger faces is made the table.
The old method, which is still in use, for roughly fashioning diamonds is
that known as bruting, from the French word, brutage, for the process, or as
shaping. Two stones of about the same size are selected, and are firmly
attached by means of a hard cement to the ends of two holders, which are

held one in each hand, and rubbed hard, one against the other, until surfaces
of the requisite size are developed on each stone. During the process the
stones are held over a small box, which catches the precious powder. A fine
sieve at the bottom of the box allows the powder to fall through into a tray
underneath, but holds back anything larger. By means of two vertical pins
placed one on each side of the box the holders are retained more easily in
the desired position, and the work is thrown mainly on the thumbs. This
work continued day after day has a very disfiguring effect upon the hands
despite the thick gloves that are worn to protect them; the skin of the
thumbs grows hard and horny, and the first and second fingers become
swollen and distorted. When the surfaces have thus been formed, the stone
is handed to the polisher, who works them into the correct shape and
afterwards polishes them, the stone passing backwards and forwards several
times between the cutter and the polisher. The table, four templets, culet and
four pavilions are first formed and polished, so that the table has a square
shape. Next the quoins are developed and polished, and finally the small
facets are polished on, not being shaped first. In modern practice the
process of bruting has been modified in some cases by the introduction of
machinery, and the facets are ground on, with considerable improvement in
the regularity of their size and disposition, and reduction in the amount of
polishing required. Moreover, to obviate the loss of material resulting from
continued grinding, large stones are first sliced by means of rapidly-
revolving copper wheels charged with diamond powder.
The laps used for polishing diamonds are made of a particular kind of
soft iron, which is found to surpass any other metal in retaining the
diamond powder. They are rotated at a high rate of speed, which is about
2000 to 2500 revolutions a minute, and the heat developed by the friction at
this speed is too great for a cement to be used; a solder or fusible alloy,
composed of one part tin to three parts lead, therefore takes its place. The
solder is held in a hollow cup of brass which is from its shape called a
‘dop,’ an old Dutch word meaning shell. Its external diameter is ordinarily
about 1½ in. (4 cm.), but larger dops are, of course, used for large stones. A
stout copper stalk is attached to the bottom of the dop; it is visible in the
view of the dop shown at e on Plate VI, and two slabs of solder are seen
lying in front of the dop. The dop containing the solder is placed in the
midst of a non-luminous flame and heated until the solder softens, when it
is removed by means of the small tongs, c, and placed upright on a stand

such as that shown at a. The long tongs, d, are used for shaping the solder
into a cone at the apex of which the diamond is placed. The solder is
worked well over the stone so that only the part to undergo polishing is
exposed. A diamond in position is shown at f. The top of the stand is saucer-
shaped to catch the stone should it accidentally fall off the dop, and to
prevent pieces of solder falling on the hand. While still hot, the dop with the
diamond in position on the solder is plunged into cold water in order to cool
it. The fact that the stone withstands this drastic treatment is eloquent
testimony to its good thermal conductivity; other gem-stones would
promptly split into fragments. It may be remarked that so high is the
temperature at which diamond burns that it may be placed in the gas flame
without any fear of untoward results. The dop is now ready for attachment
to an arm such as that shown at b; the stalk of the dop is placed in a groove
running across the split end of the arm, and is gripped tight by means of a
screw worked by the nut which is visible in the picture.

PLATE VI

APPLIANCES USED FOR POLISHING DIAMONDS.

PLATE VII

POLISHING DIAMONDS

Four such arms, each with a dop, are used with the polishing lap (Plate
VII), and each stands on two square legs on the bench. Pins, p, in pairs are
fixed to the bench to prevent the arms being carried round by the friction;
one near the lap holds the arm not far from the dop, and the other engages

in a strong metal tongue, which is best seen at the end of the arm b on Plate
VI. Though the arm, which is made of iron, is heavy, yet for polishing
purposes it is insufficient, and additional lead weights are laid on the top of
it, as in the case of the arm at the back on Plate VII. The copper stalk is
strong, yet flexible, and can be bent to suit the position of the facet to be
polished; on Plate VII the dops a and b are upright, but the other two are
inclined. In addition to the powder resulting from bruting, boart, i.e.
diamonds useless for cutting, are crushed up to supply polishing material,
and a little olive oil is used as a lubricant. Owing to the friction so much
heat is developed that even the solder would soften after a time, and
therefore, as a precaution, the dop is from time to time cooled by immersion
in water. The stone has constantly to be re-set, about six being the
maximum even of the tiny facets near the girdle that can be dealt with by
varying the inclination of the dop. As the work approaches completion the
stone is frequently inspected, lest the polishing be carried too far for the
development of the proper amount of ‘fire.’ When finished, the stones are
boiled in sulphuric acid to remove all traces of oil and dirt.
The whole operation is evidently rough and ready in the extreme; but
such amazing skill do the lapidaries acquire, that even the most careful
inspection by eye alone would scarce detect any want of proper symmetry
in a well-cut stone.
The fashioning of coloured stones, as all the gem-stones apart from
diamond are termed in the jewellery trade, is on account of their inferior
hardness a far less tedious operation. They are easily slit, for which purpose
a vertical wheel (Plate VIII) made of soft iron is used; it is charged with
diamond dust and lubricated with oil, generally paraffin. When slit to the
desired size, the stone is attached to a conveniently shaped holder by means
of a cement, the consistency of which varies with the hardness of the stone.
It is set in the cement in such a way that the plane desired for the table facet
is at right angles to the length of the holder, and the whole of the upper part
or crown is finished before the stone is removed from the cement. The
lower half or base is treated in a similar manner. Thus in the process of
grinding and polishing the stone is only once re-set; as was stated above,
diamond demands very different treatment. Again, all coloured stones are
ground down without any intermediate operation corresponding to bruting.
The holder is merely held in the hand, but to maintain its position more

exactly its other end, which is pointed, is inserted in one of the holes that
are pierced at intervals in a vertical spindle placed at a convenient distance
from the lap (Plate VIII), which one depending upon the inclination of the
facet to be formed. For hard stones, such as ruby and sapphire, diamond
powder is generally used as the abrasive agent, while for the softer stones
emery, the impure corundum, is selected; in recent years the artificially
prepared carborundum, silicide of carbon corresponding to the formula CSi,
which is harder than corundum, has come into vogue for grinding purposes,
but it is unfortunately useless for slitting, because it refuses to cling to the
wheel. To efface the scratches left by the abrasive agent and to impart a
brilliant polish to the facets, material of less hardness, such as putty-
powder, pumice, or rouge, is employed; in all cases the lubricant is water.
The grinding laps are made of copper, gun-metal, or lead; and pewter or
wooden laps, the latter sometimes faced with cloth or leather, are used for
polishing. As a general rule, the harder the stone the greater the speed of the
lap.

PLATE VIII

SLITTING COLOURED STONES

POLISHING COLOURED STONES

PLATE IX

FACETING MACHINE

As in the case of diamond, the lapidary judges of the position of the facet
entirely by eye and touch, but a skilled workman can develop a facet very
close to the theoretical position. During recent years various devices have
been invented to enable him to do his work with greater facility. A machine

of this kind is illustrated on Plate IX. The stone is attached by means of
cement to the blunt end, d, of the holder, b, which is of the customary kind,
while the other end is inserted in a hole in a wooden piece, a, which is
adjustable in height by means of the screw above it. The azimuthal
positions of the facets are arranged by means of the octagonal collar, c, the
sides of which are held successively in turn against the guide, e. The stand
itself is clamped to the bench. The machine is, however, little used except
for cheap stones, because it is too accurate and leads to waste of material.
Stones are sold by weight, and so long as the eye is satisfied, no attempt is
made to attain to absolute symmetry of shape.
The pictures on Plates X–XIII illustrate lapidaries’ workshops in various
parts of the world. The first two show an office and a workshop situated in
Hatton Garden, London; in the former certain of the staff are selecting from
the parcels stones suitable for cutting. The third depicts a more primitive
establishment at Ekaterinburg in the Urals. The fourth shows a typical
French family—père, mère, et fils—in the Jura district, all busily engaged;
on the table will be noticed a faceting machine of the kind described above.
In the fifth picture a native lapidary in Calcutta is seen at work with the
driving bow in his right, and the stone in his left, hand.
A curious difference exists in the systems of charging for cutting
diamonds and coloured stones. The cost of cutting the latter is reckoned by
the weight of the finished stone, the rate varying from 1s. to 8s. a carat
according to the character of the stone and the difficulty of the work; while
in the case of diamonds, on the other hand, the weight of the rough material
determines the cost, the rate being about 10s. to 40s. a carat according to the
size, which on the average is equivalent to about 30s. to 120s. a carat
calculated on the weight of the finished stone. The reason of the distinction
is obviously because the proper proportions in a brilliant-cut diamond must
be maintained, whatever be the loss in weight involved; in coloured stones
the shape is not of such primary importance.

PLATE X

LAPIDARY’S WORKSHOP AND OFFICE IN ENGLAND

PLATE XI

LAPIDARY’S WORKSHOP IN RUSSIA

When finished, the stone finds its way with others akin to it to the
manufacturing jeweller’s establishment, where it is handed to the setter,
who mounts it in a ring, necklace, brooch, or whatever article of jewellery it
is intended for. The metal used in the groundwork of the setting is generally

gold, but platinum is also employed where an unobtrusive and untarnishable
metal is demanded, and silver finds a place in cheaper jewellery, although it
is seriously handicapped by its susceptibility to the blackening influence of
the sulphurous fumes present in the smoke-laden atmosphere of towns. The
stone may be either embedded in the metal or held by claws. The former is
by far the safer, but the latter the more elegant, and it has the advantage of
exposing the stone à jour, to use the French jewellers’ expression, so that its
genuineness is more evidently testified. It is very important that the claw
setting be periodically examined, lest the owner one day experience the
mortification of finding that a valuable stone has dropped out; gold, owing
to its softness, wears away in course of time.
Up to quite recent years modern jewellery was justly open to the
criticism that it was lacking in variety, that little attempt was made to secure
harmonious association in either the colour or the lustre of the gem-stones,
and that the glitter of the gold mount was frequently far too obtrusive. Gold
consorts admirably with the rich glow of ruby, but is quite unsuited to the
gleaming fire of a brilliant. Where the metal is present merely for the
mechanical purpose of holding the stones in position, it should be made as
little noticeable as possible. The artistic treatment of jewellery is, however,
receiving now adequate attention in the best Paris and London houses.
Some recent designs are illustrated on Plates IV and V.

PLATE XII

FRENCH FAMILY CUTTING STONES

PLATE XIII

INDIAN LAPIDARY

CHAP T E R XI I I

NOMENCLATURE OF PRECIOUS STONES

T HE names in popular use for the principal gem-stones may be traced
back to very early times, and, since they were applied long before the
determinative study of minerals had become a science, their significance
has varied at different dates, and is even now far from precise. No
ambiguity or confusion could arise if jewellers made use of the scientific
names for the species, but most of them are unknown or at least unfamiliar
to those unversed in mineralogy, and to banish old-established names is
undesirable, even if the task were not hopeless. The name selected for a
gem-stone may have a very important bearing on its fortunes. When the
love-sick Juliet queried ‘What’s in a name?’ her mind was wandering far
from jewels; for them a name is everything. The beautiful red stones that
accompany the diamond in South Africa were almost a drug in the market
under their proper title—garnet, but command a ready sale under the
misnomer ‘Cape-ruby.’ To many minds there is a subtle satisfaction in the
possession of a stone which is assumed to be a sort of ruby that would be
destroyed by the knowledge that the stone really belonged to the cinderella
species of gem-stones—the despised garnet. For similar reasons it was
deemed advisable to offer the lustrous green garnet found some thirty and
odd years ago in the Ural Mountains as ‘olivine,’ not a happy choice since
their colour is grass- rather than olive-green, apart from the fact that the
term is in general use in science for the species known in jewellery as
peridot.
The names employed in jewellery are largely based upon the colour, the
least reliable from a determinative point of view of all the physical
characters of gem-stones. Qualifying terms are employed to distinguish
stones of obviously different hardness. ‘Oriental’ distinguishes varieties of
corundum, but does not imply that they necessarily came from the East; the
finest gem-stones originally reached Europe by that road, and the hardest
coloured stones consequently received that term of distinction.

Nearly all red stones are grouped under the name ruby, which is derived
from a Latin word, ruber, meaning red, or under other names adapted from
it, such as rubellite, rubicelle. It is properly applied to red corundum; ‘balas’
ruby is spinel, which is associated with the true ruby at the Burma mines
and is similar in appearance to it when cut, and ‘Cape’ ruby, is, as has been
stated above, a garnet from South Africa. Rubellite is the lovely rose-pink
tourmaline, fine examples of which have recently been discovered in
California, and rubicelle is a less pronouncedly red spinel. Sapphire is by
far the oldest and one of the most interesting of the words used in the
language of jewels. It occurs in Hebrew and Persian, ancient tongues, and
means blue. It was apparently employed for lapis lazuli or similar
substance, but was transferred to the blue corundum upon the discovery of
this splendid stone. Oblivious of the real meaning of the word, jewellers
apply it in a quasi-generic sense to all the varieties of corundum with the
exception of the red ruby, and give vent to such incongruous expressions as
‘white sapphire,’ ‘yellow sapphire’; it is true such stones often contain
traces of blue colour, but that is not the reason of the terms. ‘Brazilian’
sapphire is blue tourmaline, a somewhat rare tint for this species. The
curious history of the word topaz will be found below in the chapter dealing
with the species of that name. It has always denoted a yellow stone, and at
the present day is applied by jewellers indiscriminately to the true topaz and
citrine, the yellow quartz, the former, however, being sometimes
distinguished by the prefix ‘Brazilian.’ ‘Oriental’ topaz is corundum, and
‘occidental’ topaz is a term occasionally employed for the yellow quartz.
Emerald, which means green, was first used for chrysocolla, an opaque
greenish stone (p. 288), but was afterwards applied to the priceless green
variety of beryl, for which it is still retained. ‘Oriental’ emerald is
corundum, ‘Brazilian’ emerald in the eighteenth century was a common
term for the green tourmaline recently introduced to Europe, and ‘Uralian’
emerald has been tentatively suggested for the green garnet more usually
known as ‘olivine.’ Amethyst is properly the violet quartz, but with the
prefix ‘oriental’ it is also applied to violet corundum, though some jewellers
use it for the brilliant quartz, with purple and white sectors, from Siberia.
Almandine, which is derived from the name of an Eastern mart for precious
stones, has come to signify a stone of columbine-red hue, principally
garnet, but with suitable qualification corundum and spinel also.

The nomenclature of jewellery tends to suggest relations between the
gem-stones for which there is no real foundation, and to obscure the
essential identity, except from the point of view of colour, of sapphire and
ruby, emerald and aquamarine, cairngorm and amethyst.

CHAP T E R XI V

MANUFACTURED STONES

T HE initial step in the examination of a crystallized substance is to
determine its physical characters and to resolve it by chemical analysis
into its component elements; the final, and by far the hardest, step is to
build it up or synthetically prepare it from its constituents. Unknown to the
world at large, work of the latter kind has long been going on within the
walls of laboratories, and as the advance in knowledge placed in the hands
of experimenters weapons more and more comparable with those wielded
by nature, their efforts have been increasingly successful. So stupendous,
however, are the powers of nature that the possibility of reproducing, by
human agency, the treasured stones which are extracted from the earth in
various parts of the globe at the cost of infinite toil and labour has always
been derided by those ignorant of what had already been accomplished.
Great, therefore, was the consternation and the turmoil when concrete
evidence that could not be gainsaid showed that man’s restless efforts to
bridle nature to his will were not in vain, and congresses of all the high-
priests of jewellery were hastily convened to ban such unrighteous
products, with what ultimate success remains to be seen.
Crystallization may be caused in four different ways, of which the second
alone has as yet yielded stones large enough to be cut—
1. By the separation of the substance from a saturated solution. In nature
the solvent may not be merely hot water, or water charged with an acid, but
molten rock, and the temperature and the pressure may be excessively high.
2. By the solidification of the liquefied substance upon cooling. Ice is a
familiar example of this type.
3. By the sublimation of the vapour of the substance, which means the
direct passage from the vapour to the solid state without traversing the
usually intervening liquid state. It is usually the most difficult of attainment
of the four methods; the most familiar instance is snow.

4. By the precipitation of the substance from a solution when set free by
chemical action.
Other things being equal, the simpler the composition the greater is the
ease with which a substance may be expected to be formed; for, instead of
one complex substance, two or more different substances may evolve,
unless the conditions are nicely arranged. Attempts, for instance, to produce
beryl might result instead in a mixture of chrysoberyl, phenakite, and
quartz.
By far the simplest in composition of all the precious stones is diamond,
which is pure crystallized carbon; but its manufacture is attended by well-
nigh insuperable difficulties. If carbon be heated in air, it burns at a
temperature well below its melting point; moreover, unless an enormously
high pressure is simultaneously applied, the product is the other form of
crystallized carbon, namely, the comparatively worthless graphite.
Moissan’s interesting course of experiments were in some degree
successful, but the tiny diamonds were worthless as jewels, and the expense
involved in their manufacture was out of all proportion to any possible
commercial value they might have.
Next to diamond the simplest substances among precious stones are
quartz (crystallized silica) and corundum (crystallized alumina). The
crystallization of silica has been effected in several ways, but the value in
jewellery of quartz, even of the violet variety, amethyst, is not such as to
warrant its manufacture on a commercial scale. Corundum, on the other
hand, is held in high esteem; rubies and sapphires, of good colour and free
from flaws, have always commanded good prices. The question of their
production by artificial means has therefore more than academic interest.
Ever since the year 1837, when Gaudin produced a few tiny flakes,
French experimenters have steadily prosecuted their researches in the
crystallization of corundum. Frémy and Feil, in 1877, were the first to meet
with much success. A portion of one of their crucibles lined with glistening
ruby flakes is exhibited in the British Museum (Natural History).
In 1885 the jewellery market was completely taken by surprise by the
appearance of red stones, emanating, so it is alleged, from Geneva; having
the physical characters of genuine rubies, they were accepted as, and
commanded the prices of, the natural stones. It was eventually discovered

that they had resulted from the fusion of a
number of fragments of natural rubies in the oxy-
hydrogen flame. The original colour was driven
off at that high temperature, but was revived by
the previous addition of a little bichromate of
potassium. Owing to the inequalities of growth,
the cracks due to rapid cooling, the inclusion of
air-bubbles, often so numerous as to cause a
cloudy appearance, and, above all, the unnatural
colour, these reconstructed stones, as they are
termed, were far from satisfactory, but yet they
marked such an advance on anything that had
been accomplished before that for some time no
suspicion was aroused as to their being other
than natural stones.
A notable advance in the synthesis of
corundum, particularly of ruby, was made in
1904, when Verneuil, who had served his
apprenticeship to science under the guidance of
Frémy, invented his ingenious inverted form of
blowpipe (Fig. 53), which enabled him to
overcome the difficulties that had baffled earlier
Fig. 53.—Verneuil’s
Inverted Blowpipe.
investigators, and to manufacture rubies vying in
appearance after cutting with the best of nature’s
productions. The blowpipe consisted of two tubes, of which the upper, E,
wide above, was constricted below, and passing down the centre of the
lower, F, terminated just above the orifice of the latter in a fine nozzle.
Oxygen was admitted at C through the plate covering the upper end of the
tube, E. A rod, which passed through a rubber collar in the same plate,
supported inside the tube, E, a vessel, D, and at the upper end terminated in
a small plate, on which was fixed a disc, B. The hammer, A, when lifted by
the action of an electromagnet and released, fell by gravity and struck the
disc. The latter could be turned about a horizontal axis placed eccentrically,
so that the height through which the hammer fell and the consequent force
of the blow could be regulated. The rubber collar, which was perfectly gas-
tight, held the rod securely, but allowed the shocks to be transmitted to the
vessel, D, an arrangement of guides maintaining the slight motion of the

vessel strictly vertical. This vessel, which carried the alumina powder used
in the manufacture of the stone, had as its base a cylindrical sieve of fine
mesh. The succession of rapid taps of the hammer caused a regular feed of
powder down the tube, the amount being regulated by varying the height
through which the hammer fell. Hydrogen or coal-gas was admitted at G
into the outer tube, F, and in the usual way met the oxygen just above the
orifice, L. To exclude irregular draughts, the flame was surrounded by a
screen, M, which was provided with a mica window, and a water-jacket, K,
protected the upper part of the apparatus from excessive heating.
The alumina was precipitated from a solution of pure
ammonia—alum, (NH4)2SO4.Al2(SO4)3.24H2O, in distilled
water by the addition of pure ammonia, sufficient chrome-
alum also being dissolved with the ammonia-alum to
Fig. 54.—‘Boule,’ furnish about 2½ per cent. of chromic oxide in the resulting
or Pear-shaped stone. The powder, carefully prepared and purified, was
Drop.
placed, as has been stated above, in the vessel, D, and on
reaching the flame at the orifice it melted, and fell as a liquid drop, N, upon
the pedestal, P, which was formed of previously fused alumina. This
pedestal was attached by a platinum sleeve to an iron rod, Q, which was
provided with the necessary screw adjustments, R and S, for centring and
lowering it as the drop grew in size. Great care was exercised to free the
powder from any trace of potassium, which, if present, imparted a brownish
tinge to the stone. The pressure of the oxygen, low initially both to prevent
the pedestal from melting, and to keep the area of the drop in contact with
the pedestal as small as possible, because otherwise flaws tended to start on
cooling, was gradually increased until the flame reached the critical
temperature which kept the top of the drop melted, but not boiling. The
supply of powder was at the same time carefully proportioned to the
pressure. The pedestal, P, was from time to time lowered, and the drop
grew in the shape of a pear (Fig. 54), the apex of which was downwards and
adhered to the pedestal by a narrow stalk. As soon as the drop reached the
maximum size possible with the size of the flame, the gases were sharply
and simultaneously cut off. After ten minutes or so the drop was lowered
from the chamber, M, by the screw, S, and when quite cold was removed
from the pedestal.

Very few changes have been made in the method when adapted to
commercial use. Coal-gas has, however, entirely replaced the costly
hydrogen, and the hammer is operated by a cam instead of an
electromagnet, while, as may be seen from the view of a gem-stone factory
(Plate XIV), a number of blowpipes are placed in line so that their cams are
worked by the same shaft, a. The fire-clay screen, b, surrounding the flame
is for convenience of removal divided into halves longitudinally, and a
small hole is left in front for viewing the stone during growth, a red glass
screen, c, being provided in front to protect the eyes from the intense glare.
Half the fire-clay screen of the blowpipe in the centre of the Plate has been
removed to show the arrangement of the interior. The centring and the
raising and lowering apparatus, d, have been modified. The process is so
simple that one man can attend to a dozen or so of these machines, and it
takes only one hour to grow a drop large enough to be cut into a ten-carat
stone.

PLATE XIV

BLOWPIPE USED FOR THE MANUFACTURE OF RUBIES AND SAPPHIRES

The drops, unless the finished stone is required to have a similar pear
shape, are divided longitudinally through the central core into halves, which
in both shape and orientation are admirably suited to the purposes of

cutting; as a general rule, the drop splits during cooling into the desired
direction of its own accord.
Each drop is a single crystalline individual, and not,
as might have been anticipated, an alumina glass or an
irregular aggregation of crystalline fragments, and, if
the drop has cooled properly, the crystallographic axis
Fig. 55.—Bubbles
is parallel to the core of the pear. The cut stone will
and Curved Striæ in therefore have not only the density and hardness, but
Manufactured Ruby. also all the optical characters—refractivity, double
refraction, dichroism, etc.—pertaining to the natural
species, and will obey precisely the same tests with the refractometer and
the dichroscope. Were it not for certain imperfections it would be
impossible to distinguish between the stones formed in Nature’s vast
workshop and those produced within the confines of a laboratory. The
artificial stones, however, are rarely, if ever, free from minute air-bubbles
(Fig. 55), which can easily be seen with an ordinary lens. Their spherical
shape differentiates them from the plane-sided cavities not infrequently
visible in a natural stone (Fig. 56). Moreover, the colouring matter varies
slightly, but imperceptibly, in successive shells, and consequently in the
finished stone a careful eye can discern the curved striations (Fig. 55)
corresponding in shape to the original shell. In a natural stone, on the other
hand, although zones of different colours or varying shades are not
uncommon, the resulting striations are straight (Fig. 56), corresponding to
the plane faces of the original crystal form. By sacrificing material it might
be possible to cut a small stone free from bubbles, but the curved striations
would always be present to betray its origin.
The success that attended the manufacture of ruby
encouraged efforts to impart other tints to
crystallized alumina. By reducing the percentage
amount of chromic oxide, pink stones were turned
out, in colour not unlike those Brazilian topazes, the
original hue of which has been altered by the
application of heat. These artificial stones have Fig. 56.—Markings in
therefore been called ‘scientific topaz’; of course, Natural Ruby.
quite wrongly, since topaz, which is properly a fluo-
silicate of aluminium, is quite a different substance.

Early attempts made to obtain the exquisite blue tint of the true sapphire
were frustrated by an unexpected difficulty. The colouring matter, cobalt
oxide, was not diffused evenly through the drop, but was huddled together
in splotches, and it was found necessary to add a considerable amount of
magnesia as a flux before a uniform distribution of colour could be secured.
It was then discovered that, despite the colour, the stones had the physical
characters, not of sapphire, but of the species closely allied to it, namely,
spinel, aluminate of magnesium. By an unsurpassable effort of
nomenclature these blue stones were given the extraordinary name of ‘Hope
sapphire,’ from fanciful analogy with the famous blue diamond which was
once the pride of the Hope collection. A blue spinel is occasionally found in
nature, but the actual tint is somewhat different. These manufactured stones
have the disadvantage of turning purple in artificial light. By substituting
lime for magnesia as a flux, Paris, a pupil of Verneuil’s, produced blue
stones which were not affected to the same extent. The difficulty was at
length overcome at the close of 1909, when Verneuil, by employing as
tinctorial agents 0·5 per cent. of titanium oxide and 1·5 per cent. of
magnetic iron oxide, succeeded in producing blue corundum; it, however,
had not quite the tint of sapphire. Stones subsequently manufactured, which
were better in colour, contained about 0·12 per cent. of titanium oxide, but
no iron at all.
By the addition to the alumina of a little nickel oxide and vanadium oxide
respectively, yellow and yellowish green corundums have been obtained.
The latter have in artificial light a distinctly reddish hue, and have therefore
been termed ‘scientific alexandrite’; of course, quite incorrectly, since the
true alexandrite is a variety of chrysoberyl, aluminate of beryllium, a very
different substance.
If no colouring matter at all be added and the alum be free from potash,
colourless stones or white sapphires are formed, which pass under the name
‘scientific brilliant.’ It is scarcely necessary to remark that they are quite
distinct from the true brilliant, diamond.
The high prices commanded by emeralds, and the comparative success
that attended the reconstruction of ruby from fragments of natural stones,
suggested that equal success might follow from a similar process with
powdered beryl, chromic oxide being used as the colouring agent. The
resulting stones are, indeed, a fair imitation, being even provided with

flaws, but they are a beryl glass with lower specific gravity and refractivity
than the true beryl, and are wrongly termed ‘scientific emerald.’ Moreover,
recently most of the stones so named on the market are merely green paste.
It is unfortunate that the real success which has been achieved in the
manufacture of ruby and sapphire should be obscured by the ill-founded
claims tacitly asserted in other cases.
At the time the manufactured ruby was a novelty it fetched as much as £6
a carat, but as soon as it was discovered that it could easily be differentiated
from the natural stone, a collapse took place, and the price fell abruptly to
30s., and eventually to 5s. and even 1s. a carat. The sapphires run slightly
higher, from 2s. to 7s. a carat. The prices of the natural stones, which at first
had fallen, have now risen to almost their former level. The extreme
disparity at present obtaining between the prices of the artificial and the
natural ruby renders the fraudulent substitution of the one for the other a
great temptation, and it behoves purchasers to beware where and from
whom they buy, and to be suspicious of apparently remarkable bargains,
especially at places like Colombo and Singapore where tourists abound. It
is no secret that some thousands of carats of manufactured rubies are
shipped annually to the East. Caveat emptor.

CHAP T E R XV

IMITATION STONES

T HE beryl glass mentioned in the previous chapter marks the transition
stage between manufactured stones which in all essential characters are
identical with those found in nature, and artificial stones which resemble
the corresponding natural stone in outward appearance only. In a sense both
sorts may be styled artificial, but it would be misleading to confound them
under the same appellation.
Common paste,[6] which is met with in drapery goods and cheap
ornaments in general—hat-pins, buckles, and so forth—is composed of
ordinary crown-glass or flint-glass, the refractive indices being about 1·53
and 1·63 respectively. The finest quality, which is used for imitations of
brilliants, is called ‘strass.’ It is a dense lead flint-glass of high refraction
and strong colour-dispersion, consisting of 38·2 per cent. of silica, 53·3 red
lead (oxide of lead), and 7·8 potassium carbonate, with small quantities of
soda, alumina, and other substances. How admirable these imitations may
be, a study of the windows of a shop devoted to such things will show.
Unfortunately the addition of lead, which is necessary for imparting the
requisite refraction and ‘fire’ to the strass, renders the stones exceedingly
soft. All glass yields to the file, but strass stones are scratched even by
ordinary window-glass. If worn in such a way that they are rubbed, they
speedily lose the brilliance of their polish, and, moreover, they are
susceptible to attack by the sulphurous fumes present in the smoky air of
towns, and turn after a time a dirty brown in hue. When coloured stones are
to be imitated, small quantities of a suitable metallic oxide are fused with
the glass; cobalt gives rise to a royal-blue tint, chromium a ruby red, and
manganese a violet. Common paste is not highly refractive enough to give
satisfactory results when cut as a brilliant, and the bases are therefore often
coated with quicksilver, or, in the case of old jewellery, covered with foil in
the setting, in order to secure more complete reflection from the interior.
The fashioning of these imitation stones is easy and cheap. Being moulded,

they do not require cutting, and the polishing of the facets thus formed is
soon done on account of the softness of the stones.
A test with a file readily differentiates paste stones from the natural
stones they pretend to be. Being necessarily singly refractive, they are, of
course, lacking in dichroism, and their refractivity seldom accords even
approximately with that of the corresponding natural stone.
In order to meet the test for hardness the doublet was devised. Such a
stone is composed of two parts—the crown consisting of colourless quartz
or other inexpensive real and hard stone, and the base being made up of
coloured glass. When the imitation, say of a sapphire, is intended to be
more exact, the crown is made of a real sapphire, but one deficient in
colour, the requisite tint being obtained from the paste forming the under
part of the doublet. In case the base should also be tested for hardness the
triplet has been devised. In this the base is made of a real stone also, and the
coloured paste is confined to the girdle section, where it is hidden by the
setting. Sapphires and emeralds of indifferent colour are sometimes slit
across the girdle; the interior surfaces are polished, and colouring matter is
introduced with the cement, generally Canada balsam, which is used to re-
unite the two portions of the stone together. All such imitations may be
detected by placing the stone in oil, when the surfaces separating the
portions of the composite stone will be visible, or the binding cement may
be dissolved by immersing the stone, if unmounted, in boiling water, or in
alcohol or chloroform, when the stone will fall to pieces.
The glass imitations of pearls, which have become very common in
recent years, may, apart from their inferior iridescence, be detected by their
greater hardness, or by the apparent doubling of, say, a spot of ink placed on
the surface, owing to reflection from the inner surface of the glass shell.
They are made of small hollow spheres formed by blowing. Next to the
glass comes a lining of parchment size, and next the under lining, which is
the most important part of the imitation, consisting of a preparation of fish
scales called Essence d’Orient, When the lining is dry, the globe is filled
with hot wax to impart the necessary solidity. In cheap imitations the glass
balls are not lined at all, but merely heated with hydrochloric acid to give an
iridescence to the surface; sometimes they are coated with wax, which can
be scraped off with a knife.

PART II—SECTION A

PRECIOUS STONES

CHAP T E R XVI

DIAMOND

D IAMOND has held pride of place as chief of precious stones ever since
the discovery of the form of cutting known as the ‘brilliant’ revealed to
full perfection its amazing qualities; and justly so, since it combines in itself
extreme hardness, high refraction, large colour-dispersion, and brilliant
lustre. A rough diamond, especially from river gravels, has often a peculiar
greasy appearance, and is no more attractive to the eye than a piece of
washing-soda. It is therefore easy to understand why the Persians in the
thirteenth century placed the pearl, ruby, emerald, and even peridot before
it, and writers in the Middle Ages frequently esteemed it below emerald and
ruby. The Indian lapidaries, who were the first to realize that diamond could
be ground with its own powder, discovered what a wonderful difference the
removal of the skin makes in the appearance of a stone. They, however,
made no attempt to shape a stone, but merely polished the natural facets,
and only added numerous small facets when they wished to conceal flaws
or other imperfections; indeed, the famous traveller, Tavernier, from whom
most of our knowledge of early mining in India is obtained, invariably
found that a stone covered with many facets was badly flawed. The full
radiant beauty of a diamond comes to light only when it is cut in brilliant
form.
Of all precious stones diamond has the simplest composition; it is merely
crystallized carbon, another form of which is the humble and useful
graphite, commonly known as ‘black-lead.’ Surely nature has surpassed all
her marvellous efforts in producing from the same element substances with
such divergent characters as the hard, brilliant, and transparent diamond and
the soft, dull, and opaque graphite. It is, however, impossible to draw any
sharp dividing line between the two; soft diamond passes insensibly into
hard graphite, and vice versa. Boart, or bort, as it is sometimes written, is
composed of minute crystals of diamond arranged haphazardly; it possesses
no cleavage, its hardness is greater than that of the crystals, and its colour is
greyish to blackish. Carbon, carbonado, or black diamond, which is

composed of still more minute crystals, is black and opaque, and is
perceptibly harder than the crystals. It passes into graphite, which varies in
hardness, and may have any density between 2·O and 3·O. Jewellers apply
the term boart to crystals or fragments which are of no service as gems;
such pieces are crushed to powder and used for cutting and polishing
purposes.
Diamonds, when absolutely limpid and free from flaws, are said to be of
the ‘first water,’ and are most prized when devoid of any tinge of colour
except perhaps bluish (Plate I, Fig. 1). Stones with a slight tinge of yellow
are termed ‘off-coloured,’ and are far less valuable. Those of a canary-
yellow colour (Plate I, Fig. 3), however, belong to a different category, and
have a decided attractiveness. Greenish stones also are common, though it
is rare to come across one with a really good shade of that colour. Brown
stones, especially in South Africa, are not uncommon. Pink stones are less
common, and ruby-red and blue stones are rare. Those of the last-named
colour have usually what is known as a ‘steely’ shade, i.e. they are tinged
with green; stones of a sapphire blue are very seldom met with, and such
command high prices.

Figs. 57—59.—Diamond Crystals.

Diamond crystallizes (Figs. 57—59 and Plate I, Fig. 2) in octahedra with
brilliant, smooth faces, and occasionally in cubes with rough pitted faces;
sometimes three or six faces take the place of each octahedron face, and the
stone is almost spherical in shape. The surfaces of the crystals are often
marked with equilateral triangles, which are supposed to represent the
effects of incipient combustion. Twinned crystals, in which the two
individuals may be connected by a single plane or may be interpenetrating,

a star shape often resulting in the latter case, are common; sometimes, if of
the octahedron type, they are beautifully symmetrical. The rounded crystals
are frequently covered with a peculiar gum-like skin which is somewhat
less hard than the crystal itself. A large South African stone, weighing 27
grams (130 carats) and octahedral in shape, which was the gift of John
Ruskin, and named by him the ‘Colenso’ after the first bishop of Natal, is
exhibited in the British Museum (Natural History); its appearance is,
however, marred by its distinctly ‘off-coloured’ tint.
The refraction of diamond is single, but local double refraction is
common, indicating a state of strain which can often be traced to an
included drop of liquid carbonic acid; so great is the strain that many a fine
stone has burst to fragments on being removed from the ground in which it
has lain. The refractive index for the yellow light of a sodium flame is
2·4175, and the slight variation from this mean value that has been
observed, amounting only to 0·0001, testifies to the purity of the
composition. The colour-dispersion is large, being as much as 0·044, in
which respect it surpasses all colourless stones, but is exceeded by sphene
and the green garnet from the Urals (cf. p. 217). The lustre of diamond,
when polished, is so characteristic as to be termed adamantine, and is due to
the combination of high refraction and extreme hardness. Diamond is
translucent to the X (Röntgen) rays; it phosphoresces under the action of
radium, and of a high-tension electric current when placed in a vacuum
tube, and sometimes even when exposed to strong sunlight. Some diamonds
fluoresce in sunlight, turning milky, and a few even emit light when rubbed.
Crookes found that a diamond buried in radium bromide for a year had
acquired a lovely blue tint, which was not affected even by heating to
redness. The specific gravity is likewise constant, being 3·521, with a
possible variation from that mean value of 0·005; but a greater range, as
might be expected, is found in the impure boart.
Diamond is by far the hardest substance in nature, being marked 10 in
Mohs’s scale of hardness, but it varies in itself; stones from Borneo and
New South Wales are so perceptibly harder than those usually in the
lapidaries’ hands, that they can be cut only with their own and not ordinary
diamond powder, and some difficulty was experienced in cutting them
when they first came into the market. It is interesting to note that the metal
tantalum, the isolation of which in commercial amount constituted one of

the triumphs of chemistry of recent years, has about the same hardness as
diamond. Despite its extreme hardness diamond readily cleaves under a
heavy blow in planes parallel to the faces of the regular octahedron, a
property utilized for shaping the stone previous to cutting it. The fallacious,
but not unnatural, idea was prevalent up to quite modern times that a
diamond would, even if placed on an anvil, resist a blow from a hammer:
who knows how many fine stones have succumbed to this illusory test? The
fact that diamond could be split was known to Indian lapidaries at the time
of Tavernier’s visit, and it would appear from De Boodt that in the sixteenth
century the cleavability of diamond was not unknown in Europe, but it was
not credited at the time and was soon forgotten. Early last century
Wollaston, a famous chemist and mineralogist, rediscovered the property,
and, so it is said, used his knowledge to some profit by purchasing large
stones, which because of their awkward shape or the presence of flaws in
the interior were rejected by the lapidaries, and selling them back again
after cleaving them to suitable forms.
It has already been remarked (p. 79) that the interval in hardness between
diamond and corundum, which comes next to it in Mohs’s scale, is
enormously greater than that between corundum and the softest of minerals.
Diamond can therefore be cut only with the aid of its own powder, and the
cutting of diamond is therefore differentiated from that of other stones, the
precious-stone trade being to a large extent divided into two distinct groups,
namely, dealers in diamonds, and dealers in all other gem-stones.
The name of the species is derived from the popular form, adiamentem,
of the Latin adamantem, itself the alliterative form of the Greek ἀδάμας,
meaning the unconquerable, in allusion not merely to the great hardness but
also to the mistaken idea already mentioned. Boart probably comes from
the Old-French bord or bort, bastard.
At the present day diamonds are usually cut as brilliants, though the
contour of the girdle may be circular, oval, or drop-shaped to suit the
particular purpose for which the stone is required, or to keep the weight as
great as possible. Small stones for bordering a large coloured stone may
also be cut as roses or points. A perfect brilliant has 58 facets, but small
stones may have not more than 44, and exceptionally large stones may with
advantage have many more; for instance, on the largest stone cut from the
Cullinan diamond there are no fewer than 74 facets.

The description of the properties of diamond would not be complete
without a reference to the other valuable, if utilitarian, purposes to which it
is put. Without its aid much of modern engineering work and mining
operations would be impossible except at the cost of almost prohibitive
expenditure of time and money.
Boring through solid rock has been greatly facilitated by the use of the
diamond drill. For this purpose carbonado or black diamond is more
serviceable than single crystals, and the price of the former has
consequently advanced from a nominal figure up to £3 to £12 a carat. The
actual working part of the drill consists of a cast-steel ring. The crown of it
has a number of small depressions at regular intervals into which the
carbonados are embedded. On revolution of the drill an annular ring is cut,
leaving a solid core which can be drawn to the surface. For cooling the drill
and for washing away the detritus water is pumped through to the working
face. The duration of the carbonados depends on the nature of the rock and
the skill of the operator. The most troublesome rock is a sandstone or one
with sharp differences in hardness, because the carbonados are liable to be
torn out of their setting. An experienced operator can tell by the feel of the
drill the nature of the rock at the working face, and by varying the pressure
can mitigate the risk of damage to the drill.
The tenacity of diamond renders it most suitable for wire-drawing. The
tungsten filaments used in many of the latest forms of incandescent electric
lamps are prepared in this manner.
Diamond powder is used for cutting and turning the hardened steel
employed in modern armaments and for other more peaceful purposes.
Although nearly all the gem-stones scratch glass, diamond alone can be
satisfactorily employed to cut it along a definite edge. Any flake at random
will not be suitable, because it will tear the glass and form a jagged edge.
The best results are given by the junction of two edges which do not meet in
too obtuse an angle; two edges of the rhombic dodecahedron meet the
requirements admirably. The stones used by the glaziers are minute in size,
being not much larger than a pin’s head, and thirty of them on an average go
to the carat. They are set in copper or brass. Some little skill is needed to
obtain the best results.

The value of a diamond has always been determined largely by the size
of the stone, the old rule being that the rate per carat should be multiplied
by the square of the weight in carats; thus, if the rate be £10, the cost of a
two-carat stone is four times this sum, or £40, of a three-carat stone £90,
and so on. For a century, from 1750 to 1850, the rate remained almost
constant at £4 for rough, £6 for rose-cut, and £8 for brilliant-cut diamonds.
Since the latter date, owing to the increase in the supply of gold, the growth
of the spending power of the world, and the gradual falling off in the
productiveness of the Brazilian fields, the rate steadily increased about 10
per cent. each year, until in 1865 the rate for brilliants was £18. The rise
was checked by the discovery of the South African mines; moreover. since
comparatively large stones are plentiful in these mines, the rule of the
increase in the price of a stone by the square of its weight no longer holds.
The rate for the most perfect stones still remains high, because such are not
so common in the South African mines. The classification[7] adopted by the
syndicate of London diamond merchants who place upon the market the
output of the De Beers group of mines is as follows:—(a) Blue-white, (b)
white, (c) silvery Cape, (d) fine Cape, (e) Cape, (f) fine bywater, (g)
bywater, (h) fine light brown, (i) light brown, (j) brown, (k) dark brown.
Bywaters or byes are stones tinged with yellow.
The rate per carat for cut stones in the blue-white and the bywater groups
is:—
Blue-White. Bywater.
5-carat stone £40–60 £20–25
1 „ 30–40 10–15
½ „ 20–25 8–12
¼ „ 15–18 6–10
Mêlée 12–15 5–8
Mêlée are stones smaller than a quarter of a carat. It will be noticed that the
prices depart largely from the old rule; thus taking the rate for a carat blue-
white stone, the price of a five-carat stone should be from £150–200 a carat,
and for a quarter-carat stone only £7, 10s. to £10 a carat. There happens to
be at the time of writing very little demand for five-carat stones. Of course,
the prices given are subject to constant fluctuation depending upon the
supply and demand, and the whims of fashion.

CHAP T E R XVI I

OCCURRENCE OF DIAMOND

T HE whole of the diamonds known in ancient times were obtained from
the so-called Golconda mines in India. Golconda itself, now a deserted
fortress near Hyderabad, was merely the mart where the diamonds were
bought and sold. The diamond-bearing district actually spread over a wide
area on the eastern side of the Deccan, extending from the Pinner River in
the Madras Presidency northwards to the Rivers Son and Khan, tributaries
of the Ganges, in Bundelkhand. The richest mines, where the large
historical stones were found, are in the south, mostly near the Kistna River.
The diamonds were discovered in sandstone, or conglomerate, or the sands
and gravels of river-beds. The mines were visited in the middle of the
seventeenth century by the French traveller and jeweller, Tavernier, when
travelling on a commission for Louis XIV, and he afterwards published a
careful description of them and of the method of working them. The mines
seem to have been exhausted in the seventeenth century; at any rate, the
prospecting, which has been spasmodically carried on during the last two
centuries, has proved almost abortive. With the exception of the Koh-i-nor,
all the large Indian diamonds were probably discovered not long before
Tavernier’s visit. The diamonds known to Pliny, and in his time, were quite
small, and it is doubtful if any stones of considerable size came to light
before a.d. 1000.
India enjoyed the monopoly of supplying the world’s demand for
diamonds up to the discovery, in 1725, of the precious stone in Brazil.
Small stones were detected by the miners in the gold washings at Tejuco,
about eighty miles (129 km.) from Rio de Janeiro, in the Serro do Frio
district of the State of Minas Geraes. The discovery naturally caused great
excitement. So many diamonds were found that in 1727 something like a
slump took place in their value. In order to keep up prices, the Dutch
merchants, who mainly controlled the Indian output, asserted that the
diamonds had not been found in Brazil at all, but were inferior Indian
stones shipped to Brazil from Goa. The tables were neatly turned when

diamonds were actually shipped from Brazil to Goa, and exported thence to
Europe as Indian stones. This course and the continuous development of the
diamond district in Brazil rendered it impossible to hoodwink the world
indefinitely. The drop in prices was, however, stayed by the action of the
Portuguese government, who exacted such heavy duties and imposed such
onerous conditions that finally no one would undertake to work the mines.
Accordingly, in 1772 diamond-mining was declared a royal monopoly in
Brazil, and such it remained until the severance of Brazil from Portugal in
1834, when private mining was permitted by the new government subject to
the payment of reasonable royalties. The industry was enormously
stimulated by the discovery, in 1844, of the remarkably rich fields in the
State of Bahia, especially at Serra da Cincorá, where carbonado, or black
diamond, first came to light, but after a few years, owing to the difficulties
of supplying labour, the unhealthiness of the climate, and the high cost of
living, the yield fell off and gradually declined, until the importance of the
fields was finally eclipsed by the rise of the South African mines. The
Brazilian mines have proved very productive, but chiefly in small
diamonds, stones above a carat in weight being few in comparison. The
largest stone, to which the name, the Star of the South, was applied,
weighed in the rough 254½ carats; it was discovered at the Bagagem mines
in 1853. The quality of the diamonds is good, many of them having the
highly-prized bluish-white colour. The principal diamond-bearing districts
of Brazil centre at Diamantina, as Tejuco was re-named after the discovery
of diamonds, Grão Magor, and Bagagem in the State of Minas Geraes, at
Diamantina in the State of Bahia, and at Goyãz and Matto Grosso in the
States of the same names. The diamonds occur chiefly in cascalho, a gravel,
containing large masses of quartz and small particles of gold, which is
supposed to be derived from a quartzose variety of micaceous slate known
as itacolumite. The mines are now to some extent being worked by
systematic dredging of the river-beds.
Early in 1867 the children of a Boer farmer, Daniel Jacobs, who dwelt
near Hopetown on the banks of the Orange River, picked up in the course of
play near the river a white pebble, which was destined not only to mark the
commencement of a new epoch in the record of diamond mines, but to
change the whole course of the history of South Africa. This pebble
attracted the attention of a neighbour, Schalk van Niekerk, who suspected
that it might be of some value, and offered to buy it. Mrs. Jacobs, however,

gave it him, laughingly scouting the idea of accepting money for a mere
pebble. Van Niekerk showed it to a travelling trader, by name John
O’Reilly, who undertook to obtain what he could for it on condition that
they shared the proceeds. Every one he met laughed to scorn the idea that
the stone had any value, and it was once thrown away and only recovered
after some search in a yard, but at length he showed it to Lorenzo Boyes,
the Acting Civil Commissioner at Colesberg, who, from its extreme
hardness, thought it might be diamond and sent it to the mineralogist, W.
Guybon Atherston, of Grahamstown, for determination. So uncertain was
Boyes of its value that he did not even seal up the envelope containing it,
much less register the package. Atherston found immediately that the long-
scorned pebble was really a fine diamond, weighing 213/16 carats, and with
O’Reilly’s consent he submitted it to Sir Philip Wodehouse, Governor at the
Cape. The latter purchased it at once for £500, and dispatched it to be
shown at the Paris Exhibition of that year. It did not, however, attract much
attention; chimerical tales of diamond finds in remote parts of the world are
not unknown. Indeed, for some time only a few small stones were picked up
beside the Orange River, and no one believed in the existence of any
extensive diamond deposit. However, all doubt as to the advisibility of
prospecting the district was settled by the discovery of the superb diamond,
afterwards known as the ‘Star of South Africa,’ which was picked up in
March 1869 by a shepherd boy on the Zendfontein farm near the Orange
River. Van Niekerk, on the alert for news of further discoveries, at once
hurried to the spot and purchased the stone from the boy for five hundred
sheep, ten oxen, and a horse, which seemed to the boy untold wealth, but
was not a tithe of the £11,200 which Lilienfeld Bros., of Hopetown, gave
Van Niekerk.

PLATE XV

KIMBERLEY MINE, 1871

PLATE XVI

KIMBERLEY MINE, 1872

This remarkable discovery attracted immediate attention to the
potentialities of a country which produced diamonds of such a size, and
prospectors began to swarm into the district, gradually spreading up the
Vaal River. For some little time not much success was experienced, but at
length, early in 1870, a rich find was made at Klipdrift, now known as
Barkly West, which was on the banks of the Vaal River immediately
opposite the Mission camp at Pniel. The number of miners steadily
increased until the population on the two sides of the river included
altogether some four or five thousand people, and there was every
appearance of stability in the existing order of things. But a vast change
came over the scene upon the discovery of still richer mines lying to the
south-east and some distance from the river. The ground was actually
situated on the route traversed by parties hurrying to the Vaal River, but no
one dreamed of the wealth that lay under their feet. The first discovery was
made in August 1870 at the farm Jagersfontein, near Fauresmith in Orange
River Colony, by De Klerk, the intelligent overseer, who noticed in the dry

bed of a stream a number of garnets, and, knowing that they often
accompanied diamond, had the curiosity to investigate the point. He was
immediately rewarded by finding a fine diamond weighing 50 carats. In the
following month diamonds were discovered about twenty miles from
Klipdrift at Dutoitspan on the Dorstfontein farm, and a little later also on
the contiguous farm of Bultfontein; a diamond was actually found in the
mortar used in the homestead of the latter farm. Early in May 1871
diamonds were found about two miles away on De Beers’ farm, Vooruitzigt,
and two months later, in July, a far richer find was made on the same farm
at a spot which was first named Colesberg Kopje, the initial band of
prospectors having come from the town of that name near the Orange River,
but was subsequently known as Kimberley after the Secretary of State for
the Colonies at that time. Soon a large and prosperous town sprang up close
to the mines; it rapidly grew in size and importance, and to this day remains
the centre of the diamond-mining industry. Subsequent prospecting proved
almost blank until the discovery of the Premier or Wesselton mine on
Wesselton farm, about four miles from Kimberley, in September 1890; it
received the former name after Rhodes, who was Premier of Cape Colony
at that date. No further discovery of any importance was made until, in
1902, diamonds were found about twenty miles north-west-north of Pretoria
in the Transvaal, at the new Premier mine, now famed as the producer of
the gigantic Cullinan diamond.

PLATE XVII

KIMBERLEY MINE, 1874

PLATE XVIII

KIMBERLEY MINE, 1881

The Kimberley mines were at first known as the ‘dry diggings’ on
account of their arid surroundings in contradistinction to the ‘river
diggings’ by the Vaal. The dearth of water was at first one of the great
difficulties in the way of working the former mines, although subsequently

the accumulation of underground water at lower levels proved a great
obstacle to the working of the mines. The ‘river diggings’ were of a type
similar to that met with in India and Brazil, the diamonds occurring in a
gravelly deposit of limited thickness beneath which was barren rock, but the
Kimberley mines presented a phenomenon hitherto without precedent in the
whole history of diamond mining. The diamonds were found in a loose
surface deposit, which was easily worked, and for some time the
prospectors thought that the underlying limestone corresponded to the
bedrock of the river gravel, until at length one more curious than his fellows
investigated the yellowish ground underneath, and found to his surprise that
it was even richer than the surface layer. Immediately a rush was made back
to the deserted claims, and the mines were busier than ever. This ‘yellow
ground,’ as it is popularly called, was much decomposed and easy,
therefore, to work and sift. About fifty to sixty feet (15–18 m.) below the
surface, however, it passed into a far harder rock, which from its colour is
known as the ‘blue ground’; this also, to the unexpected pleasure of the
miners, turned out to contain diamonds. Difficulties arose as each claim, 30
by 30 Dutch feet (about 31 English feet or 9·45 metres square) in area, was
worked downwards. In the Kimberley mine (Plate XVI) access to the
various claims was secured by retaining parallel strips, 15 feet wide, each
claim being, therefore, reduced in width to 22½ feet, to form roadways
running from side to side of the mine in one direction. These, however,
soon gave way, not only because of the falling of the earth composing them,
but because they were undermined and undercut by the owners of the
adjacent claims. By the end of 1872 the last roadway had disappeared, and
the mine presented the appearance of a vast pit. In order to obtain access to
the claims without intruding on those lying between, and to provide for the
hauling of the loads of earth to the surface, an ingenious system of wire
cables in three tiers (Plate XVII) was erected, the lowest tier being
connected to the outermost claims, the second to claims farther from the
edge, and the highest to claims in the centre of the pit. The mine at that date
presented a most remarkable spectacle, resembling an enormous radiating
cobweb, which had a weird charm by night as the moonlight softly
illuminated it, and by day, owing to the perpetual ring of the flanged wheels
of the trucks on the running wires, twanged like some gigantic æolian harp.
This system fulfilled its purpose admirably until, with increasing depth of
the workings, other serious difficulties arose. Deprived of the support of the

hard blue ground, the walls of the mine tended to collapse, and additional
trouble was caused by the underground water that percolated into the mine.
By the end of 1883 the floor of the Kimberley mine was almost entirely
covered by falls of ‘reef’ (Plate XVIII), as the surrounding rocks are
termed, the depth then being about 400 feet (122 m.). In the De Beers mine,
in spite of the precaution taken to prevent falls of reef by cutting the walls
of the mine back in terraces, falls occurred continuously in 1884, and by
1887, at a depth of 350 feet (107 m.), all attempts at open working had to be
abandoned. In the Dutoitspan mine buttresses of blue ground were left,
which held back the reef for some years, but ultimately the mine became
unsafe, and in March 1886 a disastrous fall took place, in which eighteen
miners—eight white men and ten Kafirs—lost their lives. The Bultfontein
mine was worked to the great depth of 500 feet (152 m.), but falls occurred
in 1889 and put an end to open working. In all cases, therefore, the ultimate
end was the same: the floor of the mine became covered with a mass of
worthless reef, which rendered mining from above ground dangerous, and,
indeed, impossible except at prohibitive cost. It was then clearly necessary
to effect access to the diamond-bearing ground by means of shafts sunk at a
sufficient distance from the mine to remove any fear of falls of reef. For
such schemes co-operative working was absolutely essential. Plate XIX
illustrates the desolate character of the Kimberley mine above ground and
the vastness of the yawning pit, which is over 1000 feet (300 m.) in depth.

PLATE XIX

KIMBERLEY MINE AT THE PRESENT DAY

PLATE XX

WESSELTON (open) MINE

A certain amount of linking up of claims had already taken place, but,
although many men must have seen that the complete amalgamation of the
interests in each mine was imperative, two men alone had the capacity to
bring their ideas to fruition. C. J. Rhodes was the principal agent in the
formation in April 1880 of the De Beers Mining Company, which rapidly
absorbed the remaining claims in the mine, and was re-formed in 1887 as
the De Beers Consolidated Mining Company. Meantime, Barnett Isaacs,
better known by the cognomen Barnato, which had been adopted by his
brother Henry when engaged in earning his livelihood in the diamond
fields as an entertainer, had secured the major interests in the Kimberley
mine. Rhodes saw that, for effective working of the two mines by any
system of underground working, they must be under one management, but
to all suggestions of amalgamation Barnato remained deaf, and at last
Rhodes determined to secure control of the Kimberley mine at all costs. The

story of the titanic struggle between these two men forms one of the epics
of finance. Eventually, when shares in the Kimberley mine had been
boomed to an extraordinary height, and the price of diamonds had fallen as
low as 18s. a carat, Barnato gave way, and in July 1889 the Kimberley mine
was absorbed by the De Beers Company on payment of the enormous sum
of £5,338,650. Shortly afterwards they undertook the working of the
Dutoitspan and the Bultfontein mines, and in January 1896 they acquired
the Premier or Wesselton mine. The interests in the Jagersfontein mine were
in 1888 united in the New Jagersfontein Mining and Exploration Company,
and the mine is now worked also by the De Beers Company. Thus, until the
development of the new Premier mine in the Transvaal, the De Beers
Company practically controlled the diamond market. The development of
this last mine was begun so recently, and its size is so vast—the longest
diameter being half a mile—that open-cut working is likely to continue for
some years.

PLATE XXI

LOADING THE BLUE GROUND ON THE FLOORS, AND PLOUGHING IT OVER

PLATE XXII

WASHING-MACHINES FOR CONCENTRATING THE BLUE GROUND

Though varying slightly in details, the methods of working the mines are
identical in principle. From the steeply inclined shaft horizontal galleries
are run diagonally right across the mine, the vertical interval between
successive galleries being 40 feet. From each gallery side galleries are run
at right angles to it and parallel to the working face. The blue ground is
worked systematically backwards from the working face. The mass is
stoped, i.e. drilled and broken from the bottom upwards, until only a thin
roof is left. As soon as the section is worked out and the material removed,
the roof is allowed to fall in, and work is begun on the next section of the
same level; at the same time the first section on the level next below is
opened out. Thus work is simultaneously carried on in several levels, and a
vertical plane would intersect the working faces in a straight line obliquely
inclined to the vertical direction (Fig. 60). When freshly mined, the blue
ground is hard and compact, but it soon disintegrates under atmospheric
influence. Indeed, the yellow ground itself was merely decomposed blue

ground. No immediate attempt is made, therefore, to retrieve the precious
stones. The blue ground is spread on to the ‘floors’ (Plate XXI), i.e. spaces
of open veldt which have been cleared of bushes and inequalities, to the
depth of a couple of feet, and remains there for periods ranging from six
months to two years, depending on the quality of the blue ground and the
amount of rainfall. To hasten the disintegration the blue ground is
frequently ploughed over and occasionally watered, a remarkable
introduction of agricultural methods into mining operations. No elaborate
patrolling or guarding is required, because the diamonds are so sparsely,
though regularly, scattered through the mass that even of the actual workers
in the mines but few have ever seen a stone in the blue ground. When
sufficiently broken up, it is carted to the washing and concentrating
machines, by means of which the diamonds and the heavier constituents are
separated from the lighter material.

Fig. 60.—Vertical Section of Diamond Pipe, showing Tunnels and Stopes.

Formerly the diamonds were picked out from the concentrates by means
of the keen eyes of skilled natives; but the process has been vastly
simplified and the risk of theft entirely eliminated by the remarkable
discovery made in 1897 by F. Kirsten, of the De Beers Company, that of all
the heavy constituents of the blue ground diamond alone, with the
exception of an occasional corundum and zircon, which are easily sorted
out afterwards, adheres to grease more readily than to water. In this
ingenious machine, the ‘jigger’ or ‘greaser’ (Plate XXIII) as it is commonly

termed, the concentrates are washed over a series of galvanized-iron trays,
which are covered with a thick coat of grease. The trays are slightly inclined
downwards, and are kept by machinery in constant sideways motion
backwards and forwards. So accurate is the working of this device that few
diamonds succeed in getting beyond the first tray, and none progress as far
as the third, which is added as an additional precaution. The whole
apparatus is securely covered in so that there is no risk of theft during the
operation. The trays are periodically removed, and the grease is scraped off
and boiled to release the diamonds, the grease itself being used over again
on the trays. This is the first time in the whole course of extraction from the
mines that the diamonds are actually handled. The stones are now passed on
to the sorters, who separate them into parcels according to their size, shape,
and quality.

PLATE XXIII

DIAMOND-SORTING MACHINES

PLATE XXIV

KAFIRS PICKING OUT DIAMONDS

The classification at the mines is first into groups by the shape: (1) close
goods, (2) spotted stones, (3) rejection cleavage, (4) fine cleavage, (5) light
brown cleavage, (6) ordinary and rejection cleavage, (7) flats, (8) macles,
(9) rubbish, (10) boart. Close goods are whole crystals which contain no
flaws and can be cut into single stones. Spotted stones, as their name
suggests, contain spots which necessitate removal, and cleavage includes
stones which are so full of flaws that they have to be cleaved or split into
two or more stones. Flats are distorted octahedra, and macles are twinned
octahedra. Rubbish is material which can be utilized only for grinding
purposes, and boart consists of round dark stones which are invaluable for
rock-drills. These groups are afterwards graded into the following
subdivisions, depending on increasing depth of yellowish tint: (a) blue-
white, (b) first Cape, (c) second Cape, (d) first bye, (e) second bye, (f) off-
colour, (g) light yellow, (h) yellow. It is, however, only the first group that is
so minutely subdivided. After being purchased, the parcels are split up
again somewhat differently for the London market (cf. p. 136), and the

dealers re-arrange the stones according to the purpose for which they are
required. Formerly a syndicate of London merchants took the whole of the
produce of the Kimberley mines at a previously arranged price per carat,
but at the present time the diamonds are sold by certain London firms on
commission.
The products of each mine show differences in either form or colour
which enable an expert readily to recognize their origin. The old diggings
by the Vaal River yielded finer and more colourless stones than those found
in the dry diggings and the mines underlying them. The South African
diamonds, taken as a whole, are always slightly yellowish or ‘off-coloured’;
the mines are, indeed, remarkable for the number of fine and large, canary-
yellow and brown, stones produced. The Kimberley mine yields a fair
percentage of white, and a large number of twinned and yellow stones. The
yield of the De Beers mine comprises mostly tinted stones—yellow and
brown, occasionally silver capes, and very seldom stones free from colour.
The Dutoitspan mine is noted for its harvest of large yellow diamonds; it
also produces fine white cleavage and small white octahedra. The stones
found in the Bultfontein mine are small and spotted, but, on the other hand,
the yield has been unusually regular. The Premier or Wesselton mine yields
a large proportion of flawless octahedra, but, above all, a large number of
beautiful deep-orange diamonds. Of all the South African mines the
Jagersfontein in the Orange River Colony alone supplies stones of the
highly-prized blue-white colour and steely lustre characteristic of the old
Indian stones. The new Premier mine in the Transvaal is prolific, but mostly
in off-coloured and low-grade stones, the Cullinan diamond being a
remarkable exception.
To illustrate the amazing productiveness of the South African mines, it
may be mentioned that, according to Gardner F. Williams, the Kimberley
group of mines in sixteen years yielded 36 million carats of diamonds, and
the annual output of the Jagersfontein mine averages about a quarter of a
million carats, whereas the total output of the Brazil mines, for the whole of
the long period during which they have been worked, barely exceeds 13
million carats. The average yield of the South African mines, however,
perceptibly diminishes as the depth of the mines increases.
The most interesting point connected with the South African diamond
mines, viewed from the scientific standpoint, is the light that they have

thrown on the question of the origin of the diamond, which previously was
an incomprehensible and apparently insoluble problem. In the older mines,
just as at the river diggings by the Vaal, the stones are found in a gravelly
deposit that has resulted from the disintegration of the rocks through which
the adjacent river has passed, and it is clear that the diamond cannot have
been formed in situ here; it had been suspected, and now there is no doubt,
that the itacolumite rock of Brazil has consolidated round the diamonds
which are scattered through it, and that it cannot be the parent rock. The
occurrence at Kimberley is very different. These mines are funnels which
go downwards to unknown depths; they are more or less oval in section,
becoming narrower with increasing depth, and are evidently the result of
some eruptive agency. The Kimberley mine has been worked to a depth of
nearly 4000 feet (1200 m.), and no signs of a termination have as yet
appeared. The blue ground which fills these ‘pipes,’ as they are termed,
must have been forced up from below, since it is sharply differentiated from
the surrounding country rocks. This blue ground is a brecciated peridotite of
peculiar constitution, to which the well-known petrologist, Carvil Lewis,
who made a careful study of it, gave the name kimberlite. The blue colour
testifies to its richness in iron, and it is to the oxidation of the iron
constituent, that the change of colour to yellow in the upper levels is due.
Owing to the shafts that have been sunk for working the mines, the nature
of the surrounding rocks is known to some depth. Immediately below the
surface is a decomposed ferriferous basalt, about 20 to 90 feet (6–27 m.)
thick, next a black slaty shale, 200 to 250 feet (60–75 m.) thick, then 10 feet
(3 m.) of conglomerate, next 400 feet (120 m.) of olivine diabase, then
quartzite, about 400 feet (120 m.) thick, and lastly a quartz porphyry, which
has not yet been penetrated. The strata run nearly horizontal, and there are
no signs of upward bending at the pipes. The whole of the country,
including the mines, was covered with a red sandy soil, and there was
nothing to indicate the wealth that lay underneath. The action of water had
in process of time removed all signs of eruptive activity. The principal
minerals which are associated with diamond in the blue ground are
magnetite, ilmenite, chromic pyrope, which is put on the market as a gem
under the misnomer ‘Cape-ruby,’ ferriferous enstatite, which also is
sometimes cut, olivine more or less decomposed, zircon, kyanite, and mica.
The evidence produced by an examination of the blue ground and the
walls of the pipes proves that the pipes cannot have been volcanoes such as

Vesuvius. There is no indication whatever of the action of any excessive
temperature, while, on the other hand, there is every sign of the operation of
enormous pressure; the diamonds often contain liquid drops of carbonic
acid. Crookes puts forward the plausible theory that steam has been the
primary agency in propelling the diamond and its associates up into the
channel through which it has carved its way to freedom, and holds that
molten iron has been the solvent for carbon which has crystallized out as
diamond under the enormous pressures obtaining in remote depths of the
earth’s crust. It is pertinent to note that, by dissolving carbon in molten iron,
the eminent chemist, Moissan, was enabled to manufacture tiny diamond
crystals. Water trickling down from above would be immediately converted
into steam at very high pressure on coming into contact with the molten
iron, and, in its efforts to escape, the steam would drive the iron and its
precious contents, together with the adjacent rocks, upwards to the surface.
The ferriferous nature of the blue ground and the yellow tinge so common
to the diamonds lend confirmation to this theory. The process by which the
carbon was extracted from shales or other carboniferous rocks and
dissolved in iron still awaits elucidation.
Diamonds were found in New South Wales as long ago as 1851 on Turon
River and at Reedy Creek, near Bathurst, about ninety miles (145 km.) from
Sydney, but the find was of little commercial importance. A more extensive
deposit came to light in 1867 farther north at Mudgee. In 1872 diamonds
were discovered in the extreme north of the State, at Bingara near the
Queensland border. Another discovery was made in 1884 at Tingha, and
still more recently in the tin gravels of Inverell in the same region. In their
freedom from colour and absence of twinning the New South Wales
diamonds resemble the Brazilian stones. The average size is small, running
about five to the carat when cut; the largest found weighed nearly 6 carats
when cut. They are remarkable for their excessive hardness; they can be cut
only with their own dust, ordinary diamond dust making no impression.
The Borneo diamonds are likewise distinguished by their exceptional
hardness. They mostly occur by the river Landak, near Pontianak on the
west coast of the island. They are found in a layer of rather coarse gravel,
variable, but rarely exceeding a yard (1 m.), in depth, and are associated
with corundum and rutile, together with the precious metals gold and
platinum. Indeed, it is no uncommon sight to see natives wearing waistcoats

ornamented with gold buttons, in each of which a diamond is set. The
diamonds are well crystallized and generally of pure water; yellowish and
canary-yellow stones are also common, but rose-red, bluish, smoky, and
black stones are rare. They seldom exceed a carat in weight; but stones of
10 carats in weight are found, and occasionally they attain to 20 carats. In
1850 a diamond weighing 77 carats was discovered. The Rajah of Mattan is
said to possess one of the purest water weighing as much as 367 carats, but
no one qualified to pronounce an opinion regarding its genuineness has ever
seen it.
In Rhodesia small diamonds have been found in gravel beds resting on
decomposed granite near the Somabula forest, about 12 miles (19 km.) west
of Gwelo, in association with chrysoberyl in abundance, blue topaz,
kyanite, ruby, sapphire, tourmaline, and garnet.
The occurrence of diamond in German South-West Africa is very
peculiar. Large numbers of small stones are found close to the shore near
Luderitz Bay in a gravelly surface layer, which is nowhere more than a foot
in depth. They are picked by hand by natives and washed in sieves. In shape
they are generally six-faced octahedra or twinned octahedra, simple
octahedra being rare, and in size they run about four or five to the carat, the
largest stone as yet found being only 2 carats in weight. Their colour is
usually yellowish.
Several isolated finds of diamonds have been reported in California and
other parts of the United States, but none have proved of any importance.
The largest stone found weighed 23¾ carats uncut; it was discovered at
Manchester in Virginia.

CHAP T E R XVI I I

HISTORICAL DIAMONDS

T HE number of diamonds which exceed a hundred carats in weight when
cut is very limited. Their extreme costliness renders them something
more than mere ornaments; in a condensed and portable form they represent
great wealth and all the potentiality for good or ill thereby entailed, and
have played no small, if sinister, rôle in the moulding of history. In bygone
days when despotic government was universal, the possession of a splendid
jewel in weak hands but too often precipitated the aggression of a greedy
and powerful neighbour, and plunged whole countries into the horrors of a
ruthless and bloody war. In more civilized days a great diamond has often
been pledged as security for money to replenish an empty treasury in times
of stress. The ambitions of Napoleon might have received a set-back but for
the funds raised on the security of the famous Pitt diamond. The history of
such stones—often one long romance—is full of interest, but space will not
permit of more than a brief sketch here.
If we except the colossal Cullinan stone, the mines of Brazil and South
Africa cannot compare with the old mines of India as the birthplace of large
and perfect diamonds of world-wide fame.

(1) Koh-i-nor
The history of the famous stone called the Koh-i-nor, meaning Mound of
Light, is known as far back as the year 1304, when it fell into the hands of
the Mogul emperors, and legend even traces it back some four thousand
years previously. It remained at Delhi until the invasion of North-West India
by Nadir Shah in 1739, when it passed together with an immense amount of
spoil into the hands of the conqueror. At his death the empire which he had
so strenuously founded fell to pieces, and the great diamond after many
vicissitudes came into the possession of Runjit Singh at Lahore. His
successors kept it until upon the fall of the Sikh power in 1850 it passed to
the East India Company, in whose name it was presented by Lord Dalhousie
to Queen Victoria. At this date the stone still retained its original Indian

form, but in 1862 it was re-cut into the
form of a shallow brilliant (Fig. 62), the
weight thereby being reduced from
1861/16 to 1061/16 carats. The wisdom of
this course has been severely criticized;
the stone has not the correct shape of a
brilliant and is deficient in ‘fire,’ and it
has with the change in shape lost much
of its old historical interest. The Koh-i-
nor is the private property of the
English Royal Family, the stone shown
Fig. 61.—Koh-i-nor (top view).
in
the Tower being a model. It is valued at
£100,000.

Fig. 62.—Koh-i-nor (side view).

(2) Pitt or Regent
This splendid stone was discovered in 1701
at the famous diamond mines at Partial, on the
Kistna, about 150 miles (240 km.) from
Golconda, and weighed as much as 410 carats
in the rough. By devious ways it came into the
hands of Jamchund, a Parsee merchant, from
whom it was purchased by William Pitt,
governor of Fort St. George, Madras, for
£20,400. On his return to England Pitt had it
cut into a perfect brilliant (Fig. 63), weighing
163⅞ carats, the operation occupying the Fig. 63.—Pitt or Regent (top view).
space of two years and costing £5000; more than £7000 is said to have been
realized from the sale of the fragments left over. Pitt had an uneasy time and
lived in constant dread of theft of the stone until, in 1717, after lengthy
negotiations, he parted with it to the Duc d’Orléans, Regent of France, for
the immense sum of three and three-quarter million francs, about £135,000.
With the remainder of the French regalia it was stolen from the Garde-

meuble on August 17, 1792, in the early days of
the French Revolution, but was eventually
restored by the thieves, doubtless because of the
impossibility of disposing of such a stone, at least
intact, and it is now exhibited in the Apollo
Gallery of the Louvre at Paris. It measures about
30 millimetres in length, 25 in width, and 19 in
Fig. 64.—Pitt or Regent (side depth, and is valued at £480,000.
view).

(3) Orloff

Fig. 65.—Orloff (top view). Fig. 66.—Orloff (side view).

One of the finest diamonds existing, this large stone forms the top of the
imperial sceptre of Russia. It is rose-cut (Fig. 65), the base being a cleavage
face, and weighs 194¾ carats. It is said to have formed at one time one of
the eyes of a statue of Brahma which stood in a temple on the island of
Sheringham in the Cavery River, near Trichinopoli, in Mysore, and to have
been stolen by a French soldier who had somehow persuaded the priests to
appoint him guardian of the temple. He sold it for £2000 to the captain of
an English ship, who disposed of it to a Jewish dealer in London for
£12,000. It changed hands to a Persian merchant, Raphael Khojeh, who
eventually sold it to Prince Orloff for, so it is said, the immense sum of
£90,000 and an annuity of £4000. It was presented by Prince Orloff to
Catherine II of Russia.

(4) Great Mogul

This, the largest Indian diamond known, was found in the Kollur mines,
about the year 1650. Its original weight is said to have been 787½ carats,
but it was so full of flaws that the Venetian, Hortensio Borgis, then in India,
in cutting it to a rose form reduced its weight to 240 carats. It was seen by
Tavernier at the time of his visit to India, but it has since been quite lost
sight of. It has been identified with both the Koh-i-nor and the Orloff, and it
is even suggested that both these stones were cut from it.

(5) Sancy
The history of this diamond is very involved, and probably two or more
stones have been confused. It may have been the one cut by Berquem for
Charles the Bold, from whose body on the fatal day of Nancy, in 1477, it
was snatched by a marauding soldier. It was acquired by Nicholas Harlai,
Seigneur de Sancy, who sold it to Queen Elizabeth at the close of the
sixteenth century. A hundred years later, in 1695, it was sold by James II to
Louis XIV. The stone in the French regalia, according to the inventory
taken in 1791, weighed 53¾ carats. It was never recovered after the theft of
the regalia in the following year, but may be identical with the diamond
which was in the possession of the Demidoff family and was sold by Prince
Demidoff in 1865 to a London firm who were said to have been acting for
Sir Jamsetjee Jeejeebhoy, a wealthy Parsee of Bombay. It was shown at the
Paris Exhibition of 1867. It was almond-shaped, and covered all over with
tiny facets by Indian lapidaries.

(6) Great Table
This mysterious stone was seen by Tavernier at Golconda in 1642, but
has quite disappeared. It weighed 2423/16 carats.

(7) Moon of the Mountains
This diamond is often confused with the Orloff. It was captured by Nadir
Shah at Delhi, and after his murder was stolen by an Afghan soldier who
disposed of it to an Armenian, by name Shaffrass. It was finally acquired by
the Russian crown for an enormous sum.

(8) Nizam

A large diamond, weighing 340 carats, belonged to the Nizam of
Hyderabad; it was fractured at the beginning of the Indian Mutiny. Whether
the weight is that previous to fracture or not, there seems to be no
information.

(9) Darya-i-nor
This fine diamond, rose-cut and 186 carats in weight, is of the purest
water and merits its title of ‘River of Light.’ It seems to have been captured
by Nadir Shah at Delhi, and is now the largest diamond in the Persian
collection.

(10) Shah
This fine stone, of the purest water, was presented to the Czar Nicholas
by the Persian prince Chosroes, younger son of Abbas Mirza, in 1843. At
that time it still retained three cleavage faces which were engraved with the
names of three Persian sovereigns, and weighed 95 carats. It was, however,
subsequently re-cut with the loss of 9 carats, and the engraving has
disappeared in the process.

(11) Akbar Shah, or Jehan Ghir Shah
Once the property of the great Mogul, Akbar, this diamond was engraved
on two faces with Arabic inscriptions by the instructions of his successor,
Jehan. It disappeared, but turned up again in Turkey under the name of
‘Shepherd’s Stone’; it still retained its original inscriptions and was thereby
recognized. In 1866 it was re-cut, the weight being reduced from 116 to 71
carats, and the inscriptions destroyed. The stone was sold to the Gaekwar of
Baroda for 3½ lakhs of rupees (about £23,333).

(12) Polar Star
A beautiful, brilliant-cut stone, weighing 40 carats, which is known by
this name, is in the Russian regalia.

(13) Nassak
The Nassak diamond, which weighed 89¾ carats, formed part of the
Deccan booty, and was put up to auction in London in July 1837. It was

purchased by Emanuel, a London jeweller, who for £7200 shortly
afterwards sold it to the Duke of Westminster, in whose family it still
remains. It was originally pear-shaped, but was re-cut to a triangular form
with a reduction in weight to 78⅝ carats.

(14) Napoleon
This diamond was purchased by Napoleon Buonaparte for £8000, and
worn by him at his wedding with Josephine Beauharnais in 1796.

(15) Cumberland
This stone, which weighs 32 carats, was purchased by the city of London
for £10,000 and presented to the Duke of Cumberland after the battle of
Culloden; it is now in the possession of the Duke of Brunswick.

(16) Pigott
A fine Indian stone, weighing 47½ carats, this diamond was brought to
England by Lord Pigott in 1775 and sold for £30,000. It came into the
possession of Ali Pacha, Viceroy of Egypt, and was by his orders destroyed
at his death.

(17) Eugénie
This fine stone, weighing 51 carats, was given by the Czarina Catherine
II of Russia to her favourite, Potemkin. It was purchased by Napoleon III as
a bridal gift for his bride, and on his downfall was bought by the Gaekwar
of Baroda.

(18) White Saxon
Square in contour, measuring 11/12 in. (28 mm.), and weighing 48¾ carats,
this stone was purchased by Augustus the Strong for a million thalers (about
£150,000).

(19) Pacha of Egypt
This 40-carat brilliant was purchased by Ibrahim, Viceroy of Egypt, for
£28,000.

(20) Star of Este
Though a comparatively small stone, in weight 25½ carats, it is noted for
its perfection of form and quality. It belongs to the Archduke Franz
Ferdinand of Austrian-Este, eldest son of the Archduke Karl Ludwig.

(21) Tuscany, or Austrian Yellow
The beauty of this large stone, 133¾ carats in weight, is marred by the
tinge of yellow, which is sufficiently pronounced to impair its brilliancy; it
is a double rose in form. At one time the property of the Grand Dukes of
Tuscany, it is now in the possession of the Emperor of Austria. King
mentions a tale that it was bought at a curiosity stall in Florence for an
insignificant sum, the stone being supposed to be only yellow quartz.

(22) Star of the South
This, the largest of the Brazilian diamonds, was discovered at the mines
of Bagagem in July 1853. Perfectly transparent and without tint, it was
dodecahedral in shape and weighed 254½ carats, and was sold in the rough
for £40,000. It was cut as a perfect brilliant, being reduced in weight to
125½ carats.

(23) English Dresden
This beautiful stone, which weighed 119½ carats in the rough, was found
at the Bagagem mines, in Brazil, in 1857, and came into the possession of
Mr. E. Dresden. It was cut as a long, egg-shaped brilliant, weighing 76½
carats.

(24) Star of South Africa
The first considerable stone to be found in South Africa, it was
discovered at the Vaal River diggings in 1869, and weighed 83½ carats in
the rough. It was cut to a triangular brilliant of 46½ carats. It was finally
purchased by the Countess of Dudley for £25,000.

(25) Stewart

This large diamond, weighing in the rough 288⅜ carats, was found at the
Vaal River diggings in 1872, and was first sold for £6000 and shortly
afterwards for £9000; it was reduced on cutting to 120 carats. Like many
South African stones, it has a faint yellowish tinge.

(26) Porter-Rhodes
This blue-white stone, which weighed 150 carats, was found in a claim
belonging to Mr. Porter-Rhodes in the Kimberley mine in February 1880.

(27) Imperial, Victoria, or Great White
This large diamond weighed as much as 457 carats in the rough, and 180
when cut; it is quite colourless. It was brought to Europe in 1884, and was
eventually sold to the Nizam of Hyderabad for £20,000.

(28) De Beers
A pale yellowish stone, weighing 428½ carats, was found in the De Beers
mine in 1888. It was cut to a brilliant weighing 228½ carats, and was sold
to an Indian prince. A still larger stone of similar tinge, weighing 503¼
carats, was discovered in 1896, and among other large stones supplied by
the same mine may be mentioned one of 302 carats found in 1884, and
another of 409 carats found in early years.

(29) Excelsior
This, which prior to the discovery of the ‘Cullinan,’ was by far the largest
South African stone, was found in the Jagersfontein mine on June 30, 1893;
bluish-white in tint, it weighed 969½ carats. From it were cut twenty-one
brilliants, the larger stones weighing 67⅞, 4513/16, 4511/16, 393/16, 34, 27⅞,
25⅝, 2311/16, 1611/32, 13½ carats respectively, and the total weight of the cut
stones amounting to 3643/32 carats.

(30) Jubilee
Another large stone was discovered in the Jagersfontein mine in 1895. It
weighed 634 carats in the rough, and from it was obtained a splendid,
faultless brilliant weighing 239 carats. It was shown at the Paris Exhibition
of 1900.

(31) Star of Africa, or Cullinan
All diamonds pale into
insignificance when compared with
the colossal stone that came to light
at the Premier mine near Pretoria in
the Transvaal on January 25, 1905. It
was first called the ‘Cullinan’ after
Sir T. M. Cullinan, chairman of the
Premier Diamond Mine (Transvaal)
Company, but has recently, by desire
of King George V, received the
name ‘Star of Africa.’ The rough
stone weighed 621·2 grams or
3025¾ carats (about 1⅓ lb.); it
displayed three natural faces (Plate
XXV) and one large cleavage face,
and its shape suggested that it was a
portion of an enormous stone more
than double its size; it was
Fig. 67.—Cullinan No. 1.
transparent, colourless, and had only
one small flaw near the surface. This magnificent diamond was purchased
by the Transvaal Government for £150,000, and presented to King Edward
VII on his birthday, November 9, 1907.

PLATE XXV

CULLINAN DIAMOND
(Natural size)

The Cullinan was entrusted to
the famous firm, Messrs. I. J.
Asscher & Co., of Amsterdam, for
cutting on January 23, 1908, just
three years after its discovery. On
February 10 it was cleaved into
two parts, weighing respectively
1977½ and 1040½ carats, from
which the two largest stones have
been cut, one being a pendeloque
or drop brilliant in shape (Fig. 67)
and weighing 516½ carats, and the
other a square brilliant (Fig. 68)
weighing 3093/16 carats. The first Fig. 68.—Cullinan No. 2.
has been placed in the sceptre, and

the second in the crown of the regalia. Besides these there are a pendeloque
weighing 92 carats, a square-shaped brilliant 62, a heart-shaped stone 18⅜,
two marquises 89/16 and 11¼, an oblong stone 6⅝, a pendeloque 49/32, and 96
small brilliants weighing together 7⅜; the total weight of the cut stones
amounts to 10365/32 carats. The largest stone has 74 and the second 66
facets. The work was completed and the stones handed to King Edward in
November 1908.
Although the Premier mine has yielded no worthy compeer of the
Cullinan, it can, nevertheless, boast of a considerable number of large
stones which but for comparison with that giant would be thought
remarkable for their size, no fewer than seven of them having weights of
over 300 carats, viz. 511, 487¼, 458¾, 391½, 373, 348, and 334 carats.

(32) Star of Minas
This large diamond, which was found in 1911 at the Bagagem mines,
Minas Geraes, Brazil, had the shape of a dome with a flat base, and
weighed in the rough 35·875 grams (174¾ carats).

The large stone called the ‘Braganza,’ in the Portuguese regalia, which is
supposed to be a diamond, is probably a white topaz; it weighs 1680 carats.
The Mattan stone, pear-shaped and weighing 367 carats, which was found
in the Landak mines near the west coast of Borneo in 1787, is suspected to
be quartz.

COLOURED DIAMONDS

(1) Hope
The largest of coloured diamonds, the Hope, weighs 44⅛ carats, and has
a steely- or greenish-blue, and not the royal-blue colour of the glass models
supposed to represent it. It is believed to be a portion of a drop-form stone
(d’un beau violet) which was said to have been found at the Kollur mines,
and was secured by Tavernier in India in 1642 and sold by him to Louis
XIV in 1668; it then weighed 67 carats. This stone was stolen with the
remainder of the French regalia in 1792 and never recovered. In 1830 the

present stone (Fig. 69) was offered for sale by
Eliason, a London dealer, and was purchased for
£18,000 by Thomas Philip Hope, a wealthy banker
and a keen collector of gems. Probably the apex of
the original stone had been cut off, reducing it to a
nearly square stone. The slight want of symmetry of
the present stone lends confirmation to this view,
Fig. 69.—Hope. and two other blue stones are known, which,
together with the Hope, make up the weight of the original stone. At the
sale of the Hope collection at Christie’s in 1867 the blue diamond went to
America. In 1908 the owner disposed of it to Habib Bey for the enormous
sum of £80,000. It was put up to auction in Paris in 1909, and bought by
Rosenau, the Paris diamond merchant, for the comparatively small sum of
400,000 francs (about £16,000), and was sold in January 1911 to Mr.
Edward M’Lean for £60,000. The stone is supposed to bring ill-luck in its
train, and its history has been liberally embellished with fable to establish
the saying.

(2) Dresden
A beautiful apple-green diamond, faultless, and of the purest water, is
contained in the famous Green Vaults of Dresden. It weighs 40 carats, and
was purchased by Augustus the Strong in 1743 for 60,000 thalers (about
£9000).

(3) Paul I
A fine ruby-red diamond, weighing 10 carats, is included among the
Russian crown jewels.

(4) Tiffany
The lovely orange brilliant, weighing 125⅜ carats, which is in the
possession of Messrs. Tiffany & Co., the well-known jewellers of New
York, was discovered in the Kimberley mine in 1878.

CHAP T E R XI X

CORUNDUM
(Sapphire, Ruby)

R ANKING in hardness second to diamond alone, the species known to
science as corundum and widely familiar by the names of its varieties,
sapphire and ruby, holds a pre-eminent position among coloured gem-
stones. The barbaric splendour of ruby (Plate I, Fig. 13) and the glorious
hue of sapphire (Plate I, Fig. 11) are unsurpassed, and it is remarkable that
the same species should boast such different, but equally magnificent, tints.
They, however, by no means exhaust the resources of this variegated
species. Fine yellow stones (Plate I, Fig. 12), which compare with topaz in
colour and are its superior in hardness, and brilliant colourless stones,
which are unfortunately deficient in ‘fire’ and cannot therefore approach
diamond, are to be met with, besides others of less attractive hues, purple,
and yellowish, bluish, and other shades of green. Want of homogeneity in
the coloration of corundum is a frequent phenomenon; thus, the purple
stones on close examination are found to be composed of alternate blue and
red layers, and stones showing patches of yellow and blue colour are
common. Owing to the peculiarity of their interior arrangement certain
stones display when cut en cabochon a vivid six-rayed star of light (Plate I,
Fig. 15). Sapphire and ruby share with diamond, pearl, and emerald the first
rank in jewellery. They are popular stones, especially in rings; their
comparative rarity in large sizes, apart from the question of expense,
prevents their use in the bigger articles of jewellery. The front of the stones
is usually brilliant-cut and the back step-cut, but Indian lapidaries often
prefer to cover the stone with a large number of triangular facets, especially
if the stone be flawed; star-stones are cut more or less steeply en cabochon.
In composition corundum is alumina, oxide of aluminium, corresponding
to the formula Al2O3, but it usually contains in addition small quantities,
rarely more than 1 per cent., of ferric oxide, chromic oxide, and perhaps
other metallic oxides. When pure, it is colourless; the splendid tints which

are its glory have their origin in the minute traces of the other oxides
present. No doubt chromic oxide is the cause of the ruddy hue of ruby, since
it is possible, as explained above (p. 117), closely to imitate the ruby tint by
this means, but nothing approaching so large a percentage as 2½ has been
detected in a natural stone. The blue colour of sapphire may be due to
titanic oxide, and ferric oxide may be responsible for the yellow hue of the
‘oriental topaz,’ as the yellow corundum is termed. Sapphires, when of
considerable size, are rarely uniform in tint throughout the stone.
Alternations of blue and red zones, giving rise to an apparent purple or
violet tint, and the conjunction of patches of blue and yellow are common.
Perfectly colourless stones are less common, a slight bluish tinge being
usually noticeable, but they are not in much demand because, on account of
their lack of ‘fire,’ they are of little interest when cut. The tint of the red
stones varies considerably in depth; jewellers term them, when pale, pink
sapphires, but, of course, no sharp distinction can be drawn between them
and rubies. The most highly prized tint is the so-called pigeon’s blood, a
shade of red slightly inclined to purple. The prices for ruby of good colour
run from about 25s. a carat for small stones to between £60 and £80 a carat
for large stones, and still higher for exceptional rubies. The taste in
sapphires has changed of recent times. Formerly the deep blue was most in
demand, but now the lighter shade, that resembling the colour of corn-
flower, is preferred, because it retains a good colour in artificial light. Large
sapphires are more plentiful than large rubies, and prices run lower; even
for large perfect stones the rate does not exceed £30 a carat. Large and
uniform ‘oriental topazes’ are comparatively common, and realize moderate
prices, about 2s. to 30s. a carat according to quality and size. Green
sapphires are abundant from Australia, but their tint, a kind of deep sage-
green, is not very pleasing. Brown stones with a silkiness of structure are
also known.
The name of the species comes through the French corindon from an old
Hindu word, korund, of unknown significance, and arose from the
circumstance that the stones which first found their way to Europe came
from India. At the present day the word corundum is applied in commerce
to the opaque stones used for abrasive purposes, to distinguish the purer
material from emery, which is corundum mixed with magnetite and other
heavy stones of lower hardness. The origin of the word sapphire, which
means blue, has been discussed in an earlier chapter (p. 110). Jewellers use

it in a general sense for all corundum except ruby. Ruby comes from the
Latin ruber, red. The prefix ‘oriental’ (p. 111) is often used to distinguish
varieties of corundum, since it is the hardest of ordinary coloured stones
and the finest gem-stones in early days reached Europe by way of the East.
Corundum crystallizes either in six-sided prisms terminated by flat faces
(Plate I, Fig. 10), which are often triangularly marked, or with twelve
inclined faces, six above and six below, meeting in a girdle (Plate I, Fig.
14). Ruby favours the former and the other varieties the latter type. A fine
crystal of ruby—the ‘Edwardes,’ so named by the donor, John Ruskin, after
Sir Herbert Edwardes—which weighs 33·5 grams (163 carats), is exhibited
in the Mineral Gallery of the British Museum (Natural History), and is tilted
in such a way that the light from a neighbouring window falls on the large
basal face, and reveals the interesting markings that nature has engraved on
it. From its type of symmetry corundum is doubly refractive with a
direction of single refraction running parallel to the edge of the prism.
Owing to the relative purity of the chemical composition the refractive
indices are very constant; the ordinary index ranges from 1·766 to 1·774
and the extraordinary index from 1·757 to 1·765, the double refraction
remaining always the same, 0·009. The amount of colour-dispersion is
small, and therefore colourless corundum displays very little ‘fire.’ The
difference between the indices for red and blue light is, however,
sufficiently great that the base of a ruby may be left relatively thicker than
that of a sapphire to secure an equally satisfactory effect (cf. p. 98)—a point
of some importance to the lapidary, since stones are sold by weight and it is
his object to keep the weight as great as possible. When a corundum is
tested on the refractometer in white light a wide spectrum deliminates the
two portions of the field because of the smallness of the colour-dispersion
(cf p. 25). The dichroism of both ruby and sapphire is marked, the twin
colours given by the former being red and purplish-red, and by the latter
blue and yellowish-blue, the second colour in each instance corresponding
to the extraordinary ray. Tests with the dichroscope easily separate ruby and
sapphire from any other red or blue stone. This character has an important
bearing on the proper mode of cutting the stones. The ugly yellowish tint
given by the extraordinary ray of sapphire should be avoided by cutting the
stone with its table-facet at right angles to the prism edge, which is the
direction of single refraction. Whether a ruby should be treated in the same
way is a moot point. No doubt if the colour is deep, it is the best plan,

because the amount of absorption of light is thereby sensibly reduced, but
otherwise the delightful nuances distinguishing ruby are best secured by
cutting the table-facet parallel to the direction of single refraction. Yellow
corundum also shows distinct dichroism, but by a variation more of the
depth than of the tint of the colour; the phenomenon is faint compared with
the dichroic effect of a yellow chrysoberyl. The specific gravity also is very
constant, varying only from 3·95 to 4·10; sapphire is on the whole lighter
than ruby. Corundum has the symbol 9 on Mohs’s scale, but though coming
next to diamond it is a very poor second (cf. p. 79). As is usually the case,
the application of heat tends to lighten the colour of the stones: those of a
pale violet or a yellow colour lose the tint entirely, and the deep violet
stones turn a lovely rose colour. On the other hand the action of radium has,
as was shown by Bordas, an intensifying action on the colour, and even
develops it in a colourless stone. From the latter reaction it may be inferred
that often in an apparently colourless stone two or more selective influences
are at work which ordinarily neutralize one another, but, being unequally
stimulated by the action of radium, they thereupon give rise to colour. The
stellate appearance of asterias or star-stones—star-ruby and star-sapphire—
results from the regular arrangement either of numerous small channels or
of twin-lamellæ in the stone parallel to the six sides of the prisms; light is
reflected from the interior in the form of a six-rayed star (p. 38). Some
stones from Siam possess a markedly fibrous or silky structure.
The synthetical manufacture of ruby, sapphire, and other varieties of
corundum has already been described (p. 116).
Besides its use in jewellery corundum is on account of its hardness of
great service for many other purposes. Small fragments are extensively
employed for the bearing parts of the movements of watches, and both the
opaque corundum and the impure kind known as emery are in general use
for grinding and polishing softer stones, and steel and other metal-work.
The world’s supply of fine rubies is drawn almost entirely from the
famous ruby mines near Mogok, situated about 90 miles (145 km.) in a
north-easterly direction from Mandalay in Upper Burma and at an elevation
of about 4000 ft. (1200 m.) above sea-level. It is from this district that the
stones of the coveted carmine-red, the so-called ‘pigeon’s blood,’ colour are
obtained. The ruby occurs in a granular limestone or calcite in association
with the spinel of nearly the same appearance—the ‘balas-ruby,’ oriental

topaz (yellow corundum), tourmaline, and occasionally sapphire. Some
stones are found in the limestone on the sides of the hills, but by far the
largest quantity occur in the alluvial deposits, both gravel and clay, in the
river-beds; the ruby ground is locally known as ‘byon.’ The stones are as a
rule quite small, averaging only about four to the carat. Before the British
annexation of the country in 1885 the mines were a monopoly of the
Burmese sovereigns and were worked solely under royal licence. They are
known to be of great antiquity, but otherwise their early history is a
mystery. It is said that an astute king secured the priceless territory in 1597
from the neighbouring Chinese Shans in exchange for a small and
unimportant town on the Irrawaddy; if that be so, he struck an excellent
bargain. The mines were allotted to licensed miners, twin-tsas (eaters of the
mine) as they were called in the language of the country, who not only paid
for the privilege, but were compelled to hand over to the king all stones
above a certain weight. As might be anticipated this injunction caused
considerable trouble, and the royal monopolists constantly suspected the
miners of evading the regulation by breaking up stones of exceptional size;
from subsequent experience, it is probable that large stones were in reality
seldom found. Since 1887 the mines have been worked by arrangement
with the Government of India by the Ruby Mines, Ltd., an English
company. Its career has been far from prosperous, but during recent years,
in consequence of the improved methods of working the mines and of the
more generous terms afterwards accorded by the Government, greater
success has been experienced; the future is, however, to some extent
clouded by the advent of the synthetical stone, which has even made its way
out to the East.
Large rubies are far from common, and such as were discovered in the
old days were jealously hoarded by the Burmese sovereigns. According to
Streeter the finest that ever came to Europe were a pair brought over in
1875, at a time when the Burmese king was pressed for money. One, rich in
colour, was originally cushion-shaped and weighed 37 carats; the other was
a blunt drop in form and weighed 47 carats. Both were cut in London, the
former being reduced to 325/16 carats and the latter to 389/16 carats, and were
sold for £10,000 and £20,000 respectively. A colossal stone, weighing 400
carats, is reported to have been found in Burma; it was broken into three
pieces, of which two were cut and resulted in stones weighing 70 and 45
carats respectively, and the third was sold uncut in Calcutta for 7 lakhs of

rupees (£46,667). The finder of another large stone broke it into two parts,
which after cutting weighed 98 and 74 carats respectively; he attempted in
vain to evade the royal acquisitiveness, by giving up the larger stone to the
king and concealing the other. A fine stone, known by the formidable
appellation of ‘Gnaga Boh’ (Dragon Lord), weighed 44 carats in the rough
and 20 carats after cutting. Since the mines were taken over by the Ruby
Mines, Ltd., a few large stones have been discovered. A beautiful ruby was
found in the Tagoungnandaing Valley, and weighed 18½ carats in the rough
and 11 carats after cutting; perfectly clear and of splendid colour, it was
sold for £7000, but is now valued at £10,000. Another, weighing 77 carats
in the rough, was found in 1899, and was sold in India in 1904 for 4 lakhs
of rupees (£26,667). A stone, weighing 49 carats, was discovered in 1887,
and an enormous one, weighing as much as 304 carats, in 1890.
The ruby, as large as a pigeon’s egg, which is amongst the Russian
regalia was presented in 1777 to the Czarina Catherine by Gustav III of
Sweden when on a visit to St. Petersburg. The large red stone in the English
regalia which was supposed to be a ruby is a spinel (cf p. 206).
Comparatively uncommon as sapphires are in the Burma mines a
faultless stone, weighing as much as 79½ carats, has been discovered there.
Good rubies, mostly darker in colour than the Burmese stones, are found
in considerable quantity near Bangkok in Siam, Chantabun being the centre
of the trade, where, just as in Burma, they are intimately associated with the
red spinel. Because of the difference in tint and the consequent difference in
price, jewellers draw a distinction between Burma and Siam rubies; but
that, of course, does not signify any specific difference between them. Siam
is, however, most distinguished as the original home of splendid sapphires.
The district of Bo Pie Rin in Battambang produces, indeed, more than half
the world’s supply of sapphires. In the Hills of Precious Stones, such being
the meaning of the native name for the locality, a number of green
corundums are found. Siam also produces brown stones characterized by a
peculiar silkiness of structure. Rubies are found in Afghanistan at the
Amir’s mines near Kabul and also to the north of the lapis lazuli mines in
Badakshan.
The conditions in Ceylon are precisely the converse of those obtaining in
Burma; sapphire is plentiful and ruby rare in the island. They are found in

different rocks, sapphire occurring with garnet in gneiss, and ruby
accompanying spinel in limestone, but they come together in the resulting
gravels, the principal locality being the gem-district near Ratnapura in the
south of the island. The largest uncut ruby discovered in Ceylon weighed
42½ carats; it had, however, a decided tinge of blue in it. Ceylon is also
noted for the magnificent yellow corundum, ‘oriental topaz,’ or, as it is
locally called, ‘king topaz,’ which it produces.
Beautiful sapphires occur in various parts of India, but particularly in the
Zanskar range of the north-western Himalayas in the state of Kashmir,
where they are associated with brown tourmaline. Probably most of the
large sapphires known have emanated from India. By far the most gigantic
ever reported is one, weighing 951 carats, said to have been seen in 1827 in
the treasury of the King of Ava. The collection at the Jardin des Plantes
contains two splendid rough specimens; one, known as the ‘Rospoli,’ is
quite flawless and weighs 13215/16 carats, and the other is 2 inches in length
and 1½ inches in thickness. The Duke of Devonshire possesses a fine cut
stone, weighing 100 carats, which is brilliant-cut above and step-cut below
the girdle. An image of Buddha, which is cut out of a single sapphire, is
exhibited, mounted on a gold pin, in the Mineral Gallery of the British
Museum (Natural History).
For some years past a large quantity of sapphires have come into the
market from Montana, U.S.A., especially from the gem-district about
twelve miles west of Helena. The commonest colour is a bluish green,
generally pale, but blue, green, yellow and occasionally red stones are also
found; they are characterized by their almost metallic lustre. With them are
associated gold, colourless topaz, kyanite, and a beautiful red garnet which
is found in grains and usually mistaken for ruby. Rubies are also found in
limestone at Cowee Creek, North Carolina.
Blue and red corundum, of rather poor quality, has come from the
Sanarka River, near Troitsk, and from Miask, in the Government of
Orenburg, Russia, and similar stones have been known at Campolongo, St.
Gothard, Switzerland.
The prolific gem-district near Anakie, Queensland, supplies examples of
every known variety of corundum except ruby; blue, green, yellow, and
parti-coloured stones, and also star-stones, are plentiful. Leaf-green

corundum is known father south, in Victoria. The Australian sapphire is too
dark to be of much value.
Small rubies and sapphires are found in the gem-gravels near the
Somabula Forest, Rhodesia.

CHAP T E R XX

BERYL
(Emerald, Aquamarine, Morganite)

T HE species to be considered in this chapter includes the varieties
emerald and aquamarine, as well as what jewellers understand by beryl.
It has many incontestable claims on the attention of all lovers of the
beautiful in precious stones. The peerless emerald (Plate I, Fig. 5), which in
its verdant beauty recalls the exquisite lawns that grace the courts and
quadrangles of our older seats of learning, ranks to-day as the most costly
of jewels. Its sister stone, the lovely aquamarine (Plate I, Fig. 4), which
seems to have come direct from some mermaid’s treasure-house in the
depths of a summer sea, has charms not to be denied. Pliny, speaking of this
species, truly says, “There is not a colour more pleasing to the eye”; yet he
knew only the comparatively inferior stones from Egypt, and possibly from
the Ural Mountains. Emeralds are favourite ring-stones, and would, no
doubt, be equally coveted for larger articles of jewellery did not the
excessive cost forbid, and nothing could be more attractive for a central
stone than a choice aquamarine of deep blue-green hue. Emeralds are
usually step-cut, though Indian lapidaries often favour the en cabochon
form; aquamarines, on the other hand, are brilliant-cut in front and step-cut
at the back.
Beryl, to use the name by which the species is known to science, is
essentially a silicate of aluminium and beryllium corresponding to the
formula, Be3Al2(SiO3)6. The beryllia is often partially replaced by small
amounts of the alkaline earths, caesia, potash, soda, and lithia, varying from
about 1½ per cent. in beryl from Mesa Grande to nearly 5 in that from Pala
and Madagascar, and over 6, of which 3·6 is caesia, in beryl from Hebron,
Maine; also, as usual, chromic and ferric oxides take the place of a little
alumina; from 1 to 2 per cent. of water has been found in emerald. The
element beryllium was, as its name suggests, first discovered in a specimen

of this species, the discovery being made in 1798 by the chemist Vauquelin;
it is also known as glucinum in allusion to the sweet taste of its salts.
When pure, beryl is colourless, but it is rarely, if ever, free from a tinge of
blue or green. The colour is usually some shade of green—grass-green, of
that characteristic tint which is in consequence known as emerald-green, or
blue-green, yellowish green (Plate I, Fig. 6), and sometimes yellow, pink,
and rose-red. The peculiar colour of emerald is supposed to be caused by
chromic oxide, small quantities of which have been detected in it by
chemical analysis; moreover, experiment shows that glass containing the
same percentage amount of chromic oxide assumes the same splendid hue.
Emerald, on being heated, loses water, but retains its colour unimpaired,
which cannot therefore be due, as has been suggested, to organic matter.
The term aquamarine is applied to the deep sea-green and blue-green
stones, and jewellers restrict the term beryl to paler shades and generally
other colours, such as yellow, golden, and pink, but Kunz has recently
proposed the name morganite to distinguish the beautiful rose beryl such as
is found in Madagascar. The varying shades of aquamarine are due to the
influence of the alkaline earths modified by the presence of ferric oxide or
chromic oxide; the beautiful blushing hue of morganite is no doubt caused
by lithia.
The name of the species is derived from the Greek
βήρυλλος, an ancient word, the meaning of which has
been lost in the mists of time. The Greek word denoted
the same species in part as that now understood by the
name. Emerald is derived from a Persian word which
appeared in Greek as σμάραγδος, and in Latin as
smaragdus; it originally denoted chrysocolla, or similar
Fig. 70.—Emerald green stone, but was transferred upon the introduction
Crystal.
of the deep-green beryl from Upper Egypt. The name
aquamarine was suggested by Pliny’s exceedingly happy description of the
stones “which imitate the greenness of the clear sea,” although it was not
actually used by him. That emerald and beryl were one species was
suspected by Pliny, but the identity was not definitely established till about
a century ago. Morganite is named after John Pierpont Morgan.
The natural crystals have the form of a six-sided prism, and in the case of
emerald (Fig. 70, and Plate I, Fig. 8) invariably, if whole, end in a single

face at right angles to the length of the prism; aquamarines have in addition
a number of small inclined faces, and stones from both Russia and Brazil
often taper owing to the effects of corrosion. The sixfold character of the
crystalline symmetry necessarily entails that the double refraction, which is
small in amount, 0·006, is uniaxial in character, and, since the ordinary is
greater than the extraordinary refractive index, it is negative in sign. The
values of the indices range between 1·567 and 1·590, and 1·572 and 1·598
respectively, in the two cases, the pink beryl possessing the highest values.
The dichroism is distinct in the South American emerald, the twin colours
being yellowish and bluish green, but otherwise is rather faint. The specific
gravity varies between 2·69 and 2·79, and is therefore a little higher than
that of quartz. If, therefore, a beryl and a quartz be floating together in a
tube containing a suitable heavy liquid, the former will always be at a
sensibly lower level (cf. Fig. 32). The hardness varies from 7½ to 8,
emerald being a little softer than the other varieties. There is no cleavage,
but like most gem-stones beryl is very brittle, and can easily be fractured.
Stones rendered cloudy by fissures are termed ‘mossy.’ When heated before
the blowpipe beryl is fusible with difficulty; it resists the attack of
hydrofluoric acid as well as of ordinary acids.
In all probability the whole of the emeralds known in ancient times came
from the so-called Cleopatra emerald mines in Upper Egypt. For some
reason they were abandoned, and their position was so completely lost that
in the Middle Ages it was maintained that emeralds had never been found in
Egypt at all, but had come from America by way of the East. All doubts
were set at rest by the re-discovery of the mines early last century by
Cailliaud, who had been sent by the Viceroy of Egypt to search for them.
They were, however, not much worked, and after a few years were closed
again, and were re-opened only about ten years ago. The principal mines are
at Jebel Zabara and at Jebel Sikait in northern Etbai, about 10 miles (16
km.) apart and distant about 15 miles (24 km.) from the Red Sea, lying in
the range of mountains that run for a long distance parallel to the west coast
of the Red Sea and rise to over 1800 feet (550 m.) above sea-level. There
are numerous signs of considerable, but primitive, workings at distinct
periods. Both emeralds and beryls are found in micaceous and talcose
schists. The emeralds are not of very good quality, being cloudy and rather
light in colour. Finer emeralds have been found in a dark mica-schist,
together with other beryllium minerals, chrysoberyl and phenakite, and also

topaz and tourmaline on the Asiatic side of the Ural Mountains, near the
Takowaja River, which flows into the Bolshoi Reft River, one of the larger
tributaries of the Pyschma River, about fifty miles (80 km.) east of
Ekaterinburg, a town which is chiefly concerned with the mining and
cutting of gem-stones. The mine was accidentally discovered by a peasant,
who noticed a few green stones at the foot of an uprooted tree in 1830. Two
years later the mine was regularly worked, and remained open for twenty
years, when it was closed. It has recently been re-opened owing to the high
rates obtaining for emeralds. Very large crystals have been produced here,
but in colour they are much inferior to the South American stones; small
Siberian emeralds, on the other hand, are of better colour than small South
American emeralds, the latter being not so deep in tint. Emeralds have been
found in a similar kind of schist at Habachtal, in the Salzburg Alps. About
thirty years ago well-formed green stones were discovered with hiddenite at
Stony Point, Alexander County, in North Carolina, but not much gem
material has come to light.
The products of none of the mines that have just been mentioned can on
the whole compare with the beautiful stones which have come from South
America. At the time when the Spaniards grimly conquered Peru and
ruthlessly despoiled the country of the treasures which could be carried
away, immense numbers of emeralds—some of almost incredible size—
were literally poured into Spain, and eventually found their way to other
parts of Europe. These stones were known as Spanish or Peruvian emeralds,
but in all probability none of them were actually mined in Peru. Perhaps the
most extraordinary were the five choice stones which Cortez presented to
his bride, the niece of the Duke de Bejar, thereby mortally offending the
Queen, who had desired them for herself, and which were lost in 1529 when
Cortez was shipwrecked on his disastrous voyage to assist Charles V at the
siege of Algiers. All five stones had been worked to divers fantastic shapes.
One was cut like a bell with a fine pearl for a tongue, and bore on the rim,
in Spanish, “Blessed is he who created thee.” A second was shaped like a
rose, and a third like a horn. A fourth was fashioned like a fish, with eyes of
gold. The fifth, which was the most valuable and the most remarkable of all,
was hollowed out into the form of a cup, and had a foot of gold; its rim,
which was formed of the same precious metal, was engraved with the
words, “Inter natos mulierum non surrexit major.” As soon as the Spaniards
had seized nearly all the emeralds that the natives had amassed in their

temples or for personal adornment, they devoted their attention to searching
for the source of these marvels of nature, and eventually in 1558 they
lighted by accident upon the mines in what is now the United States of
Colombia, which have been worked almost continuously since that time.
Since the natives, who naturally resented the gross injustice with which
they had been treated, and penetrated the greed that prompted the actions of
the Spaniards, hid all traces of the mines, and refused to give any
information as to their position, it is possible that other emerald mines may
yet be found. The present mines are situated near the village of Muzo, about
75 miles (120 km.) north-north-west of Bogota, the capital of Colombia.
The emeralds occur in calcite veins in a bituminous limestone of Cretaceous
age. The Spaniards formerly worked the mines by driving adits through the
barren rock on the hillsides to the gem-bearing veins, but at the present day
the open cut method of working is employed. A plentiful supply of water is
available, which is accumulated in reservoirs and allowed at the proper time
to sweep the debris of barren rock away into the Rio Minero, leaving the
rock containing the emeralds exposed. Stones, of good quality, which are
suited for cutting, are locally known as canutillos, inferior stones, coarse or
ill-shaped, being called morallons.
Emerald, unlike some green stones, retains its purity of colour in artificial
light; in fact, to quote the words of Pliny, “For neither sun nor shade, nor
yet the light of candle, causeth to change and lose their lustre.” Many are
the superstitions that have been attached to it. Thus it was supposed to be
good for the eyes, and as Pliny says, “Besides, there is not a gem or
precious stone that so fully possesseth the eye, and yet never contenteth it
with satiety. Nay, if the sight hath been wearied and dimmed by intentive
poring upon anything else, the beholding of this stone doth refresh and
restore it again.” The idea that it was fatal to the eyesight of serpents
appears in Moore’s lines—
“Blinded like serpents when they gaze
Upon the emerald’s virgin blaze.”

The crystals occur attached to the limestone, and are therefore never
found doubly terminated. The crystal form is very simple, merely a
hexagonal prism with a flat face at the one end at right angles to it. They are
invariably flawed, so much so that a flawless emerald has passed into
proverb as unattainable perfection. The largest single crystal which is

known to exist at the present day is in the possession of the Duke of
Devonshire (Fig. 71). In section it is nearly a regular hexagon, about 2
inches (51 mm.) in diameter from side to side, and the length is about the
same; its weight is 276·79 grams (9¾ oz. Av., or 1347 carats). It is of good
colour, but badly flawed. It was given to the Duke of Devonshire by Dom
Pedro of Brazil, and was exhibited at the Great Exhibition of 1851. A fine,
though much smaller crystal, but of even better colour, which weighs 32·2
grams (156½ carats), and measures 1⅛ inch (28 mm.) in its widest cross-
diameter, and about the same in length, was acquired with the Allan-Greg
collection by the British Museum, and is exhibited in the Mineral Gallery of
the British Museum (Natural History). The finest cut emerald is said to be
one weighing 30 carats, which belongs to the Czar of Russia. A small, but
perfect and flawless, faceted emerald, which is set in a gold hoop, is also in
the British Museum (Natural History). It is shown, without the setting,
about actual size, on Plate I, Fig. 5.

Fig. 71.—Duke of Devonshire’s Emerald.
(Natural size.)

The ever great demand and the essentially restricted supply have forced
the cost of emeralds of good quality to a height that puts large stones
beyond the reach of all but a privileged few who have purses deep enough.
The rate per carat may be anything from £15 upwards, depending upon the
purity of the colour and the freedom from flaws, but it increases very
rapidly with the size, since flawless stones of more than 4 carats or so in
weight are among the rarest of jewels; a perfect emerald of 4 carats may
easily fetch £1600 to £2000. It seems anomalous to say that it has never
been easier to procure fine stones than during recent years, but the reason is
that the high prices prevailing have tempted owners of old jewellery to

realize their emeralds. On the other hand, pale emeralds are worth only a
nominal sum.
The other varieties of beryl are much less rare, and, since they usually
attain to more considerable, and sometimes even colossal, size, far larger
stones are obtainable. An aquamarine, particularly of good deep blue-green
colour, is a stone of great beauty, and it possesses the merit of preserving its
purity of tint in artificial light. It is a favourite stone for pendants, brooches,
and bracelets, and all purposes for which a large blue or green stone is
desired. The varying tints are said to be due to the presence of iron in
different percentages, and possibly in different states of oxidation. Unlike
emerald, the other varieties are by no means so easily recognized by their
colour. Blue aquamarines may easily be mistaken for topaz, or vice versa,
and the yellow beryl closely resembles other yellow stones, such as quartz,
topaz, or tourmaline. Stones which are colourless or only slightly tinted
command little more than the price of cutting, but the price of blue-green
stones rapidly advances with increasing depth of tint up to £2 a carat. The
enormous cut aquamarine which is exhibited in the Mineral Gallery of the
British Museum (Natural History), affords some idea of the great size such
stones reach; a beautiful sea-green in colour, it weighs 179·5 grams (875
carats), and is table-cut with an oval contour.
The splendid six-sided columns which have been discovered in various
parts of Siberia are among the most striking specimens in any large mineral
collection. The neighbourhood of Ekaterinburg in the Urals is prolific in
varieties of aquamarine; especially at Mursinka have fine stones been
found, in association with topaz, amethyst, and schorl, the black tourmaline.
Good stones also occur in conjunction with topaz at Miask in the
Government of Orenburg. It is found in the gold-washings of the Sanarka
River, in the Southern Urals, but the stones are not fitted for service as
gems. Magnificent blue-green and yellow aquamarines are associated with
topaz and smoky quartz in the granite of the Adun-Tschilon Mountains, near
Nertschinsk, Transbaikal. Stones have also been found at the Urulga River
in Siberia. Most of the bluish-green aquamarines which come into the
market at the present time have originated in Brazil, particularly in Minas
Novas, Minas Geraes, where clear, transparent stones, of pleasing colour, in
various shades, are found in the utmost profusion; beautiful yellow stones
also occur at the Bahia mines. Aquamarine was obtained in very early times

in Coimbatore District, Madras, India, and yellow beryl comes from
Ceylon. Fine blue crystals occur in the granite of the Mourne Mountains,
Ireland, but they are not clear enough for cutting purposes; similar stones
are found also at Limoges, Haute Vienne, France. Aquamarines of various
hues abound in several places in the United States, among the principal
localities being Stoneham in Maine, Haddam in Connecticut, and Pala and
Mesa Grande in San Diego County, California. The last-named state is
remarkable for the numerous stones of varying depth of salmon-pink that
have been found there. It is, however, surpassed by Madagascar, which has
recently produced splendid stones of perfect rose-red tint and of the finest
gem quality, some of them being nearly 100 carats in weight. These stones,
which have been assigned a special name, morganite (cf. supra), are
associated with tourmaline and kunzite. Pink and yellow beryls and deep
blue-green aquamarines occur in the island in quantity. The pink beryls
from California are generally pale or have a pronounced salmon tint, and
seldom approach the real rose-red colour of morganite; one magnificent
rose-red crystal, weighing nearly 9 lb. (4·05 kg.), has, however, been
recently discovered in San Diego County, California, and is now in the
British Museum (Natural History). Blue-green beryl, varying in tint from
almost colourless to an emerald-green, occurs with tin-stone and topaz
about 9 miles (14½ km.) north-east of Emmaville in New South Wales,
Australia.
Probably the largest and finest aquamarine crystal ever seen was one
found by a miner on March 28, 1910, at a depth of 15 ft. (5 m.) in a
pegmatite vein at Marambaya, near Arassuahy, on the Jequitinhonha River,
Minas Geraes, Brazil. It was greenish blue in colour, and a slightly irregular
hexagonal prism, with a flat face at each end, in form; it measured 19 in.
(48·5 cm.) in length and 16 in. (41 cm.) in diameter, and weighed 243 lb.
(110·5 kg.); and its transparency was so perfect that it could be seen
through from end to end (Plate XXVI). The crystal was transported to
Bahia, and sold for $25,000 (£5133).

PLATE XXVI

LARGE AQUAMARINE CRYSTAL (one-sixth natural size), FOUND AT MARAMBAYA,
MINAS GERAES, BRAZIL

PART II—SECTION B

SEMI-PRECIOUS STONES

CHAP T E R XXI

TOPAZ

T OPAZ is the most popular yellow stone in jewellery, and often forms
the principal stone in brooches or pendants, especially in old-fashioned
articles. It is a general idea that all yellow stones are topazes, and all
topazes are yellow; but neither statement is correct. A very large number of
yellow stones that masquerade as topaz are really the yellow quartz known
as citrine. The latter is, indeed, almost universally called by jewellers topaz,
the qualification ‘Brazilian’ being used by them to distinguish the true
topaz. Many species besides those mentioned yield yellow stones. Thus
corundum includes the beautiful ‘oriental topaz’ or yellow sapphire, and
yellow tourmalines are occasionally met with; the yellow chrysoberyl
always has a greenish tinge. Topaz is generally brilliant-cut in front and
step-cut at the back, and the table facet is sometimes rounded, but the
colourless stones are often cut as small brilliants; it takes an excellent and
dazzling polish.
Topaz is a silicate of aluminium corresponding to the formula
[Al(F,OH)]2SiO4, which was established in 1894 by Penfield and Minor as
the result of careful research. Contrary to the general idea, topaz is usually
colourless or very pale in tint. Yellow hues of different degrees, from pale to
a rich sherry tint (Plate I, Fig. 9), are common, and pure pale blue (Plate I,
Fig. 7) and pale green stones, which often pass as aquamarine, are far from
rare. Natural, red and pink, stones are very seldom to be met with. It is,
however, a peculiarity of the brownish-yellow stones from Brazil that the
colour is altered by heating to a lovely rose-pink. Curiously, the tint is not
apparent when the stone is hot, but develops as it cools to a normal
temperature; the colour seems to be permanent. Such stones are common in
modern jewellery. Although the change in colour is accompanied by some
slight rearrangement of the constituent molecules, since such stones are
invariably characterized by high refraction and pronounced dichroism, the
crystalline symmetry, however, remaining unaltered, the cause must be
attributed to some change in the tinctorial agent, probably oxidation. The

yellow stones from Ceylon, if treated in a similar manner, lose their colour
entirely. The pale yellow-brown stones from Russia fade on prolonged
exposure to strong sunlight, for which reason the superb suite of crystals
from the Urulga River, which came with the Koksharov collection to the
British Museum, are kept under cover.
The name of the species is derived from topazion (τοπάζειν, to seek), the
name given to an island in the Red Sea, which in olden times was with
difficulty located, but it was applied by Pliny and his contemporaries to the
yellowish peridot found there. The term was applied in the Middle Ages
loosely to any yellow stone, and was gradually applied more particularly to
the stone that was then more prevalent, the topaz of modern science. As has
already been pointed out (p. 111), the term is still employed in jewellery to
signify any yellow stone. The true topaz was probably included by Pliny
under the name chrysolithus.
The symmetry is orthorhombic, and the
crystals are prismatic in shape and terminated by
numerous inclined faces, and usually by a large
face perpendicular to the prism edge (Fig. 72).
Topaz cleaves with great readiness at right angles
to the prism edge; owing to its facile cleavage,
flaws are easily started, and caution must be
exercised not to damage a stone by knocking it
against hard and unyielding substances. The
dichroism of a yellow topaz is always
perceptible, one of the twin colours being
distinctly more reddish than the other, and the Fig. 72.—Topaz Crystal.
phenomenon is very marked in the case of stones
the colour of which has been artificially altered to pink. The values of the
least and the greatest of the principal indices of refraction vary from 1·615
to 1·629, and from 1·625 to 1·637, respectively, the double refraction being
about 0·010 in amount, and positive in sign. The high values correspond to
the altered stones. The specific gravity, the mean value of which is 3·55
with a variation of 0·05 on either side, is higher than would be expected
from the refractivity. A cleavage flake exhibits in convergent polarized light
a wide-angled biaxial picture, the ‘eyes’ lying outside the field of view. The
relation of the principal optical directions and the directions of single

refraction to the crystal are shown in Fig. 27. The hardness is 8 on Mohs’s
scale, and in this character it is surpassed only by chrysoberyl, corundum,
and diamond. Topaz is pyro-electric, in which respect tourmaline alone
exceeds it, and it may be strongly electrified by friction.
Although the range of refraction overlaps that of tourmaline, there is no
risk of confusion, because the latter has nearly thrice the amount of double
refraction (cf. p. 29). Apart from the difference in refraction, a yellow topaz
ought never to be confused with a yellow quartz, because the former sinks,
and the latter floats in methylene iodide. The same test distinguishes topaz
from beryl, and, indeed, from tourmaline also.
Judged by the criterion of price, topaz is not in the first rank of precious
stones. Stones of good colour and free from flaws are now, however, scarce.
Pale stones are worth very little, possibly less than 4s. a carat, but the price
rapidly advances with increase in colour, reaching 20s. for yellow, 80s. for
pink and blue stones. Since topazes are procurable in all sizes customary in
jewellery, the rates vary but slightly, if at all, with the size.
Topaz occurs principally in pegmatite dykes and in cavities in granite,
and is interesting to petrologists as a conspicuous instance of the result of
the action of hot acid vapours upon rocks rich in aluminium silicates.
Magnificent crystals have come from the extensive mining district which
stretches along the eastern flank of the Ural Mountains, and from the
important mining region surrounding Nertschinsk, in the Government of
Transbaikal, Siberia. Fine green and blue stones have been found at
Alabashka, near Ekaterinburg, in the Government of Perm, and at Miask in
the Ilmen Mountains, in the Government of Orenburg. Topazes of the rare
reddish hue have been picked out from the gold washings of the Sanarka
River, Troitsk, also in the Government of Orenburg. Splendid pale-brown
stones have issued from the Urulga River, near Nertschinsk, and good
crystals have come from the Adun-Tschilon Mountains. Kamchatka has
produced yellow, blue, and green stones. In the British Isles, beautiful sky-
blue, waterworn crystals have been found at Cairngorm, Banffshire, in
Scotland, and colourless stones in the Mourne Mountains, Ireland, and at St.
Michael’s Mount, Cornwall. Most of the topazes used in jewellery of the
present day come from either Brazil or Ceylon. Ouro Preto, Villa Rica, and
Minas Novas, in the State of Minas Geraes, are the principal localities in
Brazil. Numerous stones, often waterworn, brilliant and colourless or tinted

lovely shades of blue and wine-yellow, occur there; reddish stones also
have been found at Ouro Preto. Ceylon furnishes a profusion of yellow,
light-green, and colourless, waterworn pebbles. The colourless stones found
there are incorrectly termed by the natives ‘water-sapphire,’ and the light-
green stones are sold with beryl as aquamarines; the stones locally known
as ‘king topaz’ are really yellow corundum (cf. p. 181). Colourless crystals,
sometimes with a faint tinge of colour, have been discovered in many parts
of the world, such as Ramona, San Diego County, California, and Pike’s
Peak, Colorado, in the United States, San Luis Potosi in Mexico, and Omi
and Otami-yama in Japan.

CHAP T E R XXI I

SPINEL
(Balas-Ruby, Rubicelle)

S PINEL labours under the serious disadvantage of being overshadowed at
almost all points by its opulent and more famous cousins, sapphire and
ruby, and is not so well known as it deserves to be. The only variety which
is valued as a gem is the rose-tinted stone called balas-ruby (Plate XXVII,
Fig. 3), which is very similar to the true ruby in appearance; they are
probably often confused, especially since they are found in intimate
association in nature. Spinels of other colours are not very attractive to the
eye, and are not likely to be in much demand. Blue spinel (Plate XXVII,
Fig. 4) is far from common, but the shade is inclined to steely-blue, and is
much inferior to the superb tint of the true sapphire. Spinel is very hard and
eminently suitable for a ring-stone, but is seldom large and transparent
enough for larger articles of jewellery.
Spinel is an aluminate of magnesium corresponding to the formula
MgAl2O4, and therefore is closely akin to corundum, alumina, and
chrysoberyl, aluminate of beryllium. The composition may, however, vary
considerably owing to the isomorphous replacement of one element by
another; in particular, ferrous oxide or manganese oxide often takes the
place of some magnesia, and ferric oxide or chromic oxide is found instead
of part of the alumina. When pure, spinel is devoid of colour, but such
stones are exceedingly rare. No doubt chromic oxide is responsible for the
rose-red hue of balas-ruby, and also, when tempered by ferric oxide, for the
orange tint of rubicelle, and manganese is probably the cause of the peculiar
violet colour of almandine-spinel. It is scarcely possible to define all the
shades between blue and red that may be assumed by spinel. Stones which
are rich in iron are known as pleonaste or ceylonite; they are quite opaque,
but are sometimes used for ornamental wear.
The name of the species comes from a diminutive form of σπῖνος, a
spark, and refers to the fiery red colour of the most valued kind of spinel. It

may be noted that the Latin equivalent of the word, carbunculus, has been
applied to the crimson garnet when cut en cabochon. Balas is derived from
Balascia, the old name for Badakshan, the district from which the finest
stones were brought in mediæval times.
Spinel, like diamond,
belongs to the cubic system of
crystalline symmetry, and
occurs in beautiful octahedra,
or in flat triangular-shaped
plates (Figs. 73, 74) the girdles
of which are cleft at each
corner, these plates being
really twinned octahedra. The Figs. 73, 74.—Spinel Crystals.
refraction is, of course, single,
and there is therefore no double refraction or dichroism; this test furnishes
the simplest way of discriminating between the balas and the true ruby.
Owing to isomorphous replacement the value of the refractive index may lie
anywhere between 1·716 and 1·736. The lower values, about 1·720,
correspond to the most transparent red and blue stones; the deep violet
stones have values above 1·730. Spinel possesses little colour-dispersion, or
‘fire.’ In the same way the values of the specific gravity, even of the
transparent stones, vary between 3·5 and 3·7, but the opaque ceylonite has
values as high as 4·1. Spinel is slightly softer than sapphire and ruby, and
has the symbol 8 on Mohs’s scale, and it is scarcely inferior in lustre to
these stones. Spinel is easily separated from garnet of similar colour by its
lower refractivity. Spinels run from 10s. to £5 a carat, depending on their
colour and quality, and exceptional stones command a higher rate.
Spinel always occurs in close association with corundum. The balas and
the true ruby are mixed together in the limestones of Burma and Siam.
Curiously enough, the spinel despite its lower hardness is found in the river
gravels in perfect crystals, whereas the rubies are generally waterworn. Fine
violet and blue spinels occur in the prolific gem-gravels of Ceylon. A large
waterworn octahedron and a rough mass, both of a fine red colour, are
exhibited in the Mineral Gallery of the British Museum (Natural History),
and a beautiful faceted blue stone is shown close by.

The enormous red stone, oval in shape, which is set in front of the
English crown, is not a ruby, as it was formerly believed to be, but a spinel.
It was given to the gallant Black Prince by Pedro the Cruel after the battle
of Najera in 1367, and was subsequently worn by Henry V upon his helmet
at the battle of Agincourt. As usual with Indian-fashioned stones it is
pierced through the middle, but the hole is now hidden by a small stone of
similar colour.
The British Regalia also contains the famous stone called the Timur Ruby
or Khiraj-i-Alam (Tribute of the World), which weighs just over 352 carats,
and is the largest spinel-ruby known. It is uncut, but polished. Its history
goes back to 1398, when it was captured by the Amir Timur at Delhi. On
the wane of the Tartar empire the stone became the property of the Shahs of
Persia, until it was given by Abbas I to his friend and ally, the Mogul
Emperor, Jehangir. It remained at Delhi until, on the sack of that city by
Nadir Shah in 1739, it, together with immense booty, including the Koh-i-
nor, fell into the hands of the conqueror. Like the great diamond, it
eventually came into the possession of Runjit Singh at Lahore, and on the
annexation of the Punjab in 1850 passed to the East India Company. It was
shown at the Great Exhibition of 1851, and afterwards presented to Queen
Victoria.
Mention has been made above (p. 121) of the blue spinel which is
manufactured in imitation of the true sapphire. The artificial stone is quite
different in tint from the blue spinel found in nature.

CHAP T E R XXI I I

GARNET

T HE important group of minerals which are known under the general
name of garnet provides an apt illustration of the fact that rarity is an
essential condition if a stone is to be accounted precious. Owing to the large
quantity of Bohemian garnets, of a not very attractive shade of yellowish
red, that have been literally poured upon the market during the past half-
century the species has become associated with cheap and often ineffective
jewellery, and has acquired a stigma which completely prevents its attaining
any popularity with those professing a nice taste in gem-stones. It must not,
however, be supposed that garnet has entirely disappeared from high-class
jewellery although the name may not readily be found in a jeweller’s
catalogue. Those whose business it is to sell gem-stones are fully alive to
the importance of a name, and, as has already been remarked (p. 109), they
have been fain to meet the prejudices of their customers by offering garnets
under such misleading guises as ‘Cape-ruby,’ ‘Uralian emerald,’ or
‘olivine.’
Garnets may, moreover, figure under another name quite unintentionally.
Probably many a fine stone masquerades as a true ruby; the impossibility of
distinguishing these two species in certain cases by eye alone is perhaps not
widely recognized. An instructive instance came under the writer’s notice a
few years ago. A lady one day had the misfortune to fracture one of the
stones in a ruby ring that had been in the possession of her family for
upwards of a century, and was originally purchased of a leading firm of
jewellers in London. She took the ring to her jeweller, and asked him to
have the stone replaced by another ruby. A day or two later he sent word
that it was scarcely worth while to put a ruby in because the stones in the
ring were paste. Naturally distressed at such an opinion of a ring which had
always been held in great esteem by her family, the lady consulted a friend,
who suggested showing it to the writer. A glance was sufficient to prove that
if the ring had been in use so long the stones could not possibly be paste on
account of the excellent state of their polish, but a test with the

refractometer showed that the stones were really almandine-garnets, which
so often closely resemble the true ruby in appearance. Beautiful as the
stones were, the ring was probably not worth one-tenth what the value
would have been had the stones been rubies.
To the student of mineralogy garnet is for many reasons of peculiar
interest. It affords an excellent illustration of the facility which certain
elements possess for replacing one another without any great disturbance of
the crystalline form. Despite their apparent complexity in composition all
garnets conform to the same type of formula: lime, magnesia, and ferrous
and manganese oxides, and again alumina and ferric and chromic oxides
may replace each other in any proportion, iron being present in two states of
oxidation, and it would be rare to find a stone which agrees in composition
exactly with any of the different varieties of garnet given below.
Garnet belongs to the cubic
system of crystalline
symmetry. Its crystals are
commonly of two kinds, both
of which are very
characteristic, the regular
dodecahedron, i.e. twelve-
faced figure (Fig. 75), and the
tetrakis-octahedron or three- Figs. 75, 76.—Garnet Crystals.
faced octahedron (Fig. 76); the
latter crystals are, especially when weather- or water-worn, almost spherical
in shape. Closer and more refined observations have shown that garnet is
seldom homogeneous, being usually composed of several distinct
individuals of a lower order of symmetry. Although singly refractive as far
as can be determined with the refractometer or by deviation through a
prism, yet when examined under the polarizing microscope, garnets display
invariably a small amount of local double refraction. The transition from
light to darkness is, however, not sharp as in normal cases, but is prolonged
into a kind of twilight. In hardness, garnet is on the whole about the same as
quartz, but varies slightly; hessonite and andradite are a little softer, pyrope,
spessartite, and almandine are a little harder, while uvarovite is almost the
same. All the varieties except uvarovite are fusible when heated before the
blowpipe, and small fragments melt sufficiently on the surface in the

ordinary bunsen flame to adhere to the platinum wire holding them. This
test is very useful for separating rough red garnets, pyrope or almandine,
from red spinels or zircons of very similar appearance. Far greater variation
occurs in the other physical characters. The specific gravity may have any
value between 3·55 and 4·20, and the refractive index ranges between
1·740 and 1·890. Both the specific gravity and the refractive index increase
on the whole with the percentage amount of iron.
Garnet is a prominent constituent of many kinds of rocks, but the
material most suitable for gem purposes occurs chiefly in crystalline schists
or metamorphic limestones. Pyrope and demantoid are furnished by
peridotites and the serpentines resulting from them; almandine and
spessartite come mostly from granites.
The name of the species is derived from the Latin granatus, seed-like,
and is suggested by the appearance of the spherical crystals when embedded
in their pudding-like matrix.
The varieties most adapted to jewellery are the fiery-red pyrope and the
crimson and columbine-red almandine; the closer they approach the ruddy
hue of ruby the better they are appreciated. Hessonite was at one time in
some demand, but it inclines too much to the yellowish shade of red and
possesses too little perfection of transparency to accord with the taste of the
present day. Demantoid provides beautiful, pale and dark emerald-green
stones, of brilliant lustre and high dispersion, which are admirably adapted
for use in pendants or necklaces; on account of their comparative softness it
would be unwise to risk them in rings. In many stones the colour takes a
yellowish shade, which is less in demand. Uvarovite also occurs in
attractive emerald-green stones, but unfortunately none as yet have been
found large enough for cutting. A few truly magnificent spessartites are
known—one, a splendid example, weighing 6¾ carats, being in the
possession of Sir Arthur Church; but the species is far too seldom
transparent to come into general use. The price varies per carat from 2s. for
common garnet to 10s. for stones most akin to ruby in colour, and
exceptional demantoids may realize even as much as £10 a carat. The old
style of cutting was almost invariably rounded or en cabochon, but at the
present day the brilliant-cut front and the step-cut back is most commonly
adopted.

The several varieties will now be considered in detail.

(a) Hessonite
(Grossular, Cinnamon-Stone, Hyacinth, Jacinth)
This variety, strictly a calcium-aluminium garnet corresponding to the
formula Ca3Al2(SiO4)3, but generally containing some ferric oxide and
therefore tending towards andradite, is called by several different names. In
science it is usually termed grossular, a word derived from grossularia, the
botanical name for gooseberry, in allusion to the colour and appearance of
many crystals, or hessonite, and less correctly essonite, words derived from
the Greek ἥσσων in reference to the inferior hardness of these stones as
compared with zircon of similar colour; in jewellery it is better known as
cinnamon-stone, if a golden-yellow in colour, or hyacinth or jacinth. The
last word, which is indiscriminately used for hessonite and yellow zircon,
but should more properly be applied to the latter, is derived from an old
Indian word (cf. p. 229); jewellers, however, retain it for the garnet.
Only the yellow and orange shades of hessonite (Plate XXIX, Fig. 5) are
used for jewellery. Neither the brownish-green kind, to which the term
grossular may properly be applied, nor the rose-red is transparent enough to
serve as a gem-stone. Hessonite may mostly be recognized, even when cut,
by the curiously granular nature of its structure, just as if it were composed
of tiny grains imperfectly fused together; this appearance, which is very
characteristic, may readily be perceived if the interior of the stone be
viewed through a lens of moderate power.
The specific gravity varies from 3·55 to 3·66, and the refractive index
from 1·742 to 1·748. The hardness is on the whole slightly below that of
quartz. When heated before a blowpipe it easily fuses to a greenish glass.
The most suitable material is found in some profusion in the gem-gravels
of Ceylon, in which it is mixed up with zircon of an almost identical
appearance; both are called hyacinth. Hessonites from other localities,
although attractive as museum specimens, are not large and clear enough
for cutting purposes. Switzerland at one time supplied good stones, but the
supply has long been exhausted.

(b) Pyrope

(‘Cape-Ruby’)
Often quite ruby-red in colour (Plate XXIX, Fig. 6), this variety is
probably the most popular of the garnets. It is strictly a magnesium-
aluminium garnet corresponding to the formula Mg3Al2(SiO4)3, but usually
contains some ferrous oxide and thus approaches almandine. Both are
included among the precious garnets. Its name is derived from πυρωπός,
fire-like, in obvious allusion to its characteristic colour.
Although at its best pyrope closely resembles ruby, its appearance is
often marred by a tinge of yellow which decidedly detracts from its value.
Pyrope generally passes as a variety of ruby, and under such names as
‘Cape-ruby,’ ‘Arizona-ruby,’ depending on the origin of the stones,
commands a brisk sale. The specific gravity varies upwards from 3·70,
depending upon the percentage amount of iron present, and similarly the
refractive index varies upwards from 1·740; in the higher values pyrope
merges into almandine. Its hardness is slightly greater than that of quartz,
and may be expressed on Mohs’s scale by the symbol 7¼.
An enormous quantity of small red stones, mostly with a slight tinge of
yellow, have been brought to light at Teplitz, Aussig, and other spots in the
Bohemian Mittelgebirge, and a considerable industry in cutting and marting
them has grown up at Bilin. Fine ruby-red stones accompany diamond in
the ‘blue ground’ of the mines at Kimberley and also at the Premier mine in
the Transvaal. Similar stones are also found in Arizona and Colorado in the
United States, and in Australia, Rhodesia, and elsewhere.
Although commonly quite small in size, pyrope has occasionally attained
to considerable size. According to De Boodt the Kaiser Rudolph II had one
in his possession valued at 45,000 thalers (about £6750). The Imperial
Treasury at Vienna contains a stone as large as a hen’s egg. Another about
the size of a pigeon’s egg is in the famous Green Vaults at Dresden, and the
King of Saxony has one, weighing 468½ carats, set in an Order of the
Golden Fleece.

(c) Rhodolite
This charming pale-violet variety was found at Cowee Creek and at
Mason’s Branch, Macon County, North Carolina, U.S.A., but in too limited
amount to assume the position in jewellery it might otherwise have

expected. In composition it lies between pyrope and almandine, and may be
supposed to contain a proportion of two molecules of the former to one of
the latter. Its specific gravity is 3·84, refractive index 1·760, and hardness
7¼. It exhibits in the spectroscope the absorption-bands characteristic of
almandine.

(d) Almandine
(Carbuncle)
This variety is iron-aluminium garnet corresponding to the formula
Fe3Al2(SiO4)3, but the composition is very variable. In colour it is deep
crimson and violet or columbine-red (Plate XXIX, Fig. 8), but with
increasing percentage amount of ferric oxide it becomes brown and black,
and opaque, and quite unsuitable for jewellery. The name of the variety is a
corruption of Alabanda in Asia Minor, where in Pliny’s time the best red
stones were cut. Almandine is sometimes known as Syriam, or incorrectly
Syrian garnet, because at Syriam, once the capital of the ancient kingdom of
Pegu, which now forms part of Lower Burma, such stones were cut and
sold. Crimson stones, cut in the familiar en cabochon form and known as
carbuncles, were extensively employed for enriching metalwork, and a half-
century or so ago were very popular for ornamental wear, but their day has
long since gone. Such glowing stones are aptly described by their name,
which is derived from the Latin carbunculus, a little spark. In Pliny’s time,
however, the term was used indiscriminately for all red stones. It has
already been remarked that the word spinel has a similar significance.
The specific gravity varies from 3·90 for transparent stones to 4·20 for
the densest black stones, and the refractive index may be as high as 1·810.
Almandine is one of the hardest of the garnets, and is represented by the
symbol 7½ on Mohs’s scale. The most interesting and curious feature of
almandine lies in the remarkable and characteristic absorption-spectrum
revealed when the transmitted light is examined with a spectroscope (p. 61).
The phenomenon is displayed most vividly by the violet stones, and is,
indeed, the cause of their peculiar colour.
Although a common mineral, almandine of a quality fitted for jewellery
occurs in comparatively few localities. It is found in Ceylon, but not so
plentifully as hessonite. Good stones are mined in various parts of India,

and are nearly all cut at Delhi or Jaipur. Brazil supplies good material,
especially in the Minas Novas district of Minas Geraes, where it
accompanies topaz, and Uruguay also furnishes serviceable stones.
Almandine is found in Australia, and in many parts of the United States.
Recently small stones of good colour have been discovered at Luisenfelde
in German East Africa.

(e) Spessartite
Properly a manganese-aluminium garnet corresponding to the formula
Mn3Al2(SiO4)3, this variety generally contains iron in both states of
oxidation. If only transparent and large enough its aurora-red colour would
render it most acceptable in jewellery. Two splendid stones have, indeed,
been found in Ceylon (p. 211), and good stones rather resembling
hessonites have been quarried at Amelia Court House in Virginia, and
others have come from Nevada; otherwise, spessartite is unknown as a
gem-stone.
The specific gravity ranges from 4·0 to 4·3, and the refractive index is
about 1·81, both characters being high; the hardness is slightly greater than
that of quartz.

(f) Andradite
(Demantoid, Topazolite, ‘Olivine’)
Andradite is strictly a calcium-iron garnet corresponding to the formula
Ca3Fe2(SiO4)3, but as usual the composition varies considerably. It is
named after d’Andrada, a Portuguese mineralogist, who made a study of
garnet more than a century ago.
Once contemptuously styled common garnet, andradite suddenly sprang
into the rank of precious stones upon the discovery some thirty years ago of
the brilliant, green stones (Plate XXIX, Fig. 7) in the serpentinous rock
beside the Bobrovka stream, a tributary of the Tschussowaja River, in the
Sissersk district on the western side of the Ural Mountains. The shade of
green varies from olive through pistachio to a pale emerald, and is probably
due to chromic oxide. Its brilliant lustre, almost challenging that of
diamond, and its enormous colour-dispersion, in which respect it actually

transcends diamond, raise it to a unique position among coloured stones.
Unfortunately its comparative softness limits it to such articles of jewellery
as pendants and necklaces, where it is not likely to be rubbed. When first
found it was supposed to be true emerald, which does actually occur near
Ekaterinburg, and was termed ‘Uralian emerald.’ When analysis revealed its
true nature, it received from science the slightly inharmonious name of
demantoid in compliment to its adamantine lustre. Jewellers, however,
prefer to designate it ‘olivine,’ not very happily, because the stones usually
cut are not olive-green and the name is already in extensive use in science
for a totally distinct species (p. 225); they recognized the hopelessness of
endeavouring to find a market for them as garnets. The yellow kind of
andradite known as topazolite would be an excellent gem-stone if only it
were found large and transparent enough. Ordinary andradite is brown or
black, and opaque; it has occasionally been used for mourning jewellery.
The specific gravity varies from 3·8 to 3·9, being about 3·85 for
demantoid, which has a high refractive index, varying from 1·880 to 1·890,
and may with advantage be cut in the brilliant form. It is the softest of the
garnets, being only 6½ on Mohs’s scale.

(g) Uvarovite
This variety, which is altogether unknown in jewellery, is a calcium-iron
garnet corresponding mainly to the formula Ca3Cr2(SiO4)3, but with some
alumina always present, and was named after a Russian minister. It has an
attractive green colour, and is, moreover, hard, being about on Mohs’s scale,
but it has never yet come to light of a size suitable for cutting. The specific
gravity is low, varying from 3·41 to 3·52. Unlike the kindred varieties it
cannot be fused by heating before an ordinary blowpipe.

CHAP T E R XXI V
TOURMALINE
(Rubellite)

T OURMALINE is unsurpassed even by corundum in variety of hue, and
it has during recent years rapidly advanced in public favour, mainly
owing to the prodigal profusion in which nature has formed it in that
favoured State, California, the garden of the west. Its comparative softness
militates against its use in rings, but its gorgeous coloration renders it
admirably fitted for service in any article of jewellery, such as a brooch or a
pendant, in which a large central stone is required. Like all coloured stones
it is generally brilliant-cut in front and step-cut at the back, but occasionally
it is sufficiently fibrous in structure to display, when cut en cabochon,
pronounced chatoyancy.
The composition of this complex species has long been a vexed question
among mineralogists, but considerable light was recently thrown on the
subject by Schaller, who showed that all varieties of tourmaline may be
referred to a formula of the type 12SiO2.3B2O3.(9-x)
[(Al,Fe)2O3].3x[(Fe,Mn,Ca,Mg,K2,Na2,Li2,H2)O].3H2O. The ratios of boric
oxide, silica, and water are nearly constant in all analyses, but great
variation is possible in the proportions of the other constituents. Having
regard to this complexity, it is not surprising to find that the range in colour
is so great. Colourless stones, to which the name achroite is sometimes
given, were at one time exceedingly rare, but they are now found in greater
number in California. Stones which are most suited to jewellery purposes
are comparatively free from iron, and apparently owe their wonderful tints
to the alkaline earths; lithia, for instance, is responsible for the beautiful tint
of the highly prized rubellite, and magnesia, no doubt, for the colour of the
brown stones of various tints. Tourmaline rich in iron is black and almost
opaque. It is a striking peculiarity of the species that the crystals are rarely
uniform in colour throughout, the boundaries between the differently
coloured portions being sharp and abrupt, and the tints remarkably in

contrast. Sometimes the sections are separated by planes at right angles to
the length of the crystal, and sometimes they are zonal, bounded by
cylindrical surfaces running parallel to the same length. In the latter case a
section perpendicular to the length shows zones of at least three contrasting
tints. In the Brazilian stones the core is generally red, bounded by white,
with green on the exterior, while the reverse is the case in the Californian
stones, the core being green or yellow, bounded by white, with red on the
exterior. Tourmaline may, indeed, be found of almost every imaginable tint,
except, perhaps, the emerald green and the royal sapphire-blue. The
principal varieties are rose-red and pink (rubellite) (Plate XXVII, Fig. 1),
green (Brazilian emerald), indigo-blue (indicolite), blue (Brazilian
sapphire), yellowish green (Brazilian peridot) (Plate XXVII, Fig. 2), honey-
yellow (Ceylonese peridot), violet-red (siberite), and brown (Plate XXVII,
Fig. 8). The black, opaque stones are termed schorl.
The name of the species is derived from the Ceylonese word, turamali,
and was first employed when a parcel of gem-stones was brought to
Amsterdam from Ceylon in 1703; in Ceylon, however, the term is applied
by native jewellers to the yellow zircon commonly found in the island.
Schorl, the derivation of which is unknown, is the ancient name for the
species, and is still used in that sense by miners, but it has been restricted by
science to the black variety. The ‘Brazilian emerald’ was introduced into
Europe in the seventeenth century and was not favourably received,
possibly because the stones were too dark in colour and were not properly
cut; that they should have been confused with the true emerald is eloquent
testimony to the extreme ignorance of the characters of gem-stones
prevalent in those dark ages. Achroite comes from the Greek, ἄχροος,
without colour.
To the crystallographer tourmaline is one of the most interesting of
minerals. If the crystals, which are usually prismatic in form, are doubly
terminated, the development is so obviously different at the two ends (Fig.
77) as to indicate that directional character in the molecular arrangement,
termed the polarity, which is borne out by other physical properties.
Tourmaline is remarkably dichroic, A brown stone, except in very thin
sections, is practically opaque to the ordinary ray, and consequently a
section cut parallel to the crystallographic axis, i.e. to the length of a crystal
prismatically developed, transmits only the extraordinary ray. Such sections

were in use for yielding plane-polarized light
before Nicol devised the calcite prism known by
his name (cf. p. 44). It is evident that tourmaline,
unless very light in tint, must be cut with the
table facet parallel to that axis, because otherwise
the stone will appear dark and lifeless. The
values of the extraordinary and ordinary
refractive indices range between 1·614 and
1·638, and 1·633 and 1·669 respectively; the
double refraction, therefore, is fairly large,
amounting to 0·025, and, since the ordinary
exceeds the extraordinary ray, its character is
negative. The specific gravity varies from 3·0 to
Fig. 77.—Tourmaline 3·2. The lower values in both characters
Crystal. correspond to the lighter coloured stones used in
jewellery; the black stones, as might be expected
from their relative richness in iron, are the densest. The hardness is only
about the same as that of quartz, or perhaps a little greater, varying from 7
to 7½. It will be noticed that the range of refractivity overlaps that of topaz
(q.v.) but the latter has a much smaller double refraction, and may thus be
distinguished (p. 29). Unmounted stones are still more easily distinguished,
because tourmaline floats in methylene iodide, while topaz sinks. The pyro-
electric phenomenon (cf. p. 82) for which tourmaline is remarkable,
although of little value as a test in the case of a cut stone, is of great
scientific interest, because it is strong evidence of the peculiar crystalline
symmetry pertaining to its molecular arrangement. Tourmalines range in
price from 5s. to 20s. a carat according to their colour and quality, but
exceptional stones may command a higher rate.
Tourmaline is usually found in the pegmatite dykes of granites, but it also
occurs in schists and in crystalline limestones. Rubellite is generally
associated with the lithia mica, lepidolite; the groups of delicate pink
rubellite bespangling a background of greyish white lepidolite are among
the most beautiful of museum specimens. Magnificent crystals of pink,
blue, and green tourmaline have been found in the neighbourhood of
Ekaterinburg, principally at Mursinka, in the Urals, Russia, and fine
rubellite has come from the Urulga River, and other spots near Nertschinsk,
Transbaikal, Asiatic Russia. Elba produces pink, yellowish, and green

stones, frequently particoloured; sometimes the crystals are blackened at the
top, and are then known locally as ‘nigger-heads.’ Ceylon supplies small
yellow stones—the original tourmaline—which are confused with the
zircon of a similar colour, and rubellite accompanies the ruby at Ava,
Burma. Beautiful crystals, green and red, often diversely coloured, come
from various parts, such as Minas Novas and Arassuahy, of the State of
Minas Geraes, Brazil. Suitable gem material has been found in numerous
parts of the United States. Paris and Hebron in Maine have produced
gorgeous pink and green crystals, and Auburn in the same state has supplied
deep-blue, green, and lilac stones. Fine crystals, mostly green, but also pink
and particoloured, occur in an albite quarry near the Conn River at Haddam
Neck, Connecticut. All former localities have, however, been surpassed by
the extraordinary abundance of superb green, and especially pink, crystals
at Pala and Mesa Grande in San Diego County, California. As elsewhere,
many-hued stones are common. The latter locality supplies the more
perfectly transparent crystals. Kunz states that two remarkable rubellite
crystals were found there, one being 45 mm. in length and 42 mm. in
diameter, and the other 56 mm. in length and 24 mm. in diameter.
Madagascar, which has proved of recent years to be rich in gem-stones,
supplies green, yellow, and red stones, both uniformly tinted and
particoloured, which in beauty, though perhaps not in size, bear comparison
with any found elsewhere.

CHAP T E R XXV
PERIDOT

T HE beautiful bottle-green stone, which from its delicate tint has earned
from appreciative admirers the poetical sobriquet of the evening
emerald, and which has during recent years crept into popular favour and
now graces much of the more artistic jewellery, is named as a gem-stone
peridot—a word long in use among French jewellers, the origin and
meaning of which has been forgotten—but is known to science either as
olivine, on account of the olive-green colour sometimes characterizing it, or
as chrysolite. It is of interest to note that the last word, derived from
χρυσός, golden, and λίθος, stone, was in use at the time of Pliny, but was
employed for topaz and other yellow stones, while his topaz, curiously
enough, designated the modern peridot (cf. p. 199), an inversion that has
occurred in other words. The true olivine must not be confused with the
jewellers’ ‘olivine,’ which is a green garnet from the Ural Mountains (p.
217). Peridot is comparatively soft, the hardness varying from 6½ to 7 on
Mohs’s scale, and is suitable only for articles which are not likely to be
scratched; the polish of a peridot worn in a ring would soon deteriorate. The
choicest stones are in colour a lovely bottle-green (Plate XXIX, Fig. 2) of
various depths; the olive-green stones (Plate XXIX, Fig. 3) cannot compare
with their sisters in attractiveness. The step form of cutting is considered the
best for peridot, but it is sometimes cut round or oval in shape, with
brilliant-cut fronts.
Peridot is a silicate of magnesium and iron, corresponding to the formula
(Mg,Fe)2SiO4, ferrous iron, therefore, replacing magnesia. To the ferrous
iron it is indebted for its colour, the pure magnesium silicate being almost
colourless, and the olive tint arises from the oxidation of the iron. The
latitude in the composition resulting from this replacement is evinced in the
considerable range that has been observed in the physical characters, but the
crystalline symmetry persists unaltered; the lower values correspond to the
stones that are usually met with as gems. Peridot belongs to the
orthorhombic system of crystalline symmetry, and the crystals, which

display a large number of faces, are prismatic in form and generally
somewhat flattened. The stones, however, that come into the market for
cutting as gems are rarely unbroken. The dichroism is rather faint, one of
the twin colours being slightly more yellowish than the other, but it is more
pronounced in the olive-tinted stones. The values of the least and greatest of
the principal indices of refraction vary greatly, from 1·650 and 1·683 to
1·668 and 1·701, but the double refraction, amounting to 0·033, remains
unaffected. Peridot, though surpassed by sphene in extent of double
refraction, easily excels all the ordinary gem-stones in this respect, and this
character is readily recognizable in a cut stone by the apparent doubling of
the opposite edges when viewed through the table facet (cf. p. 41). An
equally large variation occurs in the specific gravity, namely, from 3·3 to
3·5.

PLATE XXVII

2. TOURMALINE 3. BALAS-RUBY 4. BLUE SPINEL
1. RUBELLITE

7. AMETHYST
5. QUARTZ
6. WHITE OPAL

8. TOURMALINE 10. FIRE OPAL

9. BLACK OPAL

11. ALEXANDRITE 13. ALEXANDRITE
(By daylight) 12. CHRYSOBERYL (By artificial light)

GEM-STONES

Peridots of deep bottle-green hue command moderate prices at the
present day, about 30s. a carat being asked for large stones; the paler tinted
stones run down to a few shillings a carat. The rate per carat may be very
much larger for stones of exceptional size and quality.

Olivine, to use the ordinary mineralogical term, is a common and
important constituent of certain kinds of igneous rocks, and it is also found
in those strange bodies, meteorites, which come to us from outer cosmical
space. Except in basaltic lavas, it occurs in grains and rarely in well-shaped
crystals. Stones that are large and transparent enough for cutting purposes
come almost entirely from the island Zebirget or St. John situated on the
west coast of the Red Sea, opposite to the port of Berenice. This island
belongs to the Khedive of Egypt, and is at present leased to a French
syndicate. It is believed to be the same as the mysterious island which
produced the ‘topaz’ of Pliny’s time. Magnificent stones have been
discovered here, rich green in colour, and 20 to 30, and occasionally as
much as 80, carats in weight when cut; a rough mass attained to the large
weight of 190 carats. Pretty, light-green stones are supplied by Queensland,
and peridots of a less pleasing dark-yellowish shade of green, and without
any sign of crystal form, have during recent years come from North
America. Stones rather similar to those from Queensland have latterly been
found in the Bernardino Valley in Upper Burma, not far from the ruby
mines.

CHAP T E R XXVI
ZIRCON
(Jargoon, Hyacinth, Jacinth)

Z IRCON, which, if known at all in jewellery, is called by its variety
names, jargoon and hyacinth or jacinth, is a species that deserves
greater recognition than it receives. The colourless stones rival even
diamond in splendour of brilliance and display of ‘fire’; the leaf-green
stones (Plate XXIX, Fig. 13) possess a restful beauty that commends itself;
the deep-red stones (Plate XXIX, Fig. 14), if somewhat sombre, have a
certain grandeur; and no other species produces such magnificent stones of
golden-yellow hue (Plate XXIX, Fig. 12). Zircon is well known in Ceylon,
which supplies the world with the finest specimens, and is highly
appreciated by the inhabitants of that sunny isle, but it scarcely finds a place
in jewellery elsewhere. The colourless stones are cut as brilliants, but
brilliant-cut fronts with step-cut backs is the usual style adopted for the
coloured stones.
Zircon is a silicate of zirconium corresponding to the formula ZrSiO4, but
uranium and the rare earths are generally present in small quantities. The
aurora-red variety is known as hyacinth or jacinth, and the term jargoon is
applied to the other transparent varieties, and especially to the yellow
stones. The most attractive colours shown by zircon are leaf-green, golden-
yellow, and deep red. Other common colours are brown, greenish, and sky-
blue. Colourless stones are not found in nature, but result from the
application of heat to the yellow and brown stones.
The name of the species is ancient, and comes from the Arabic zarqūn,
vermilion, or the Persian zarqūn, gold-coloured. From the same source in
all probability is derived the word jargoon through the French jargon and
the Italian giacone. Hyacinth (cf. p. 211) is transliterated from the Greek
ὑάκινθος, itself adapted from an old Indian word; it is in no way connected
with the flower of the same name. The last word has seen some changes of
meaning. In Pliny’s time yellow zircons were indiscriminately classified

with other yellow stones as chrysolite. His hyacinth was used for the
sapphire of the present day, but was subsequently applied to any transparent
corundum. Upon the introduction of the terms, sapphire and ruby, for the
blue and the red corundum hyacinth became restricted to the other varieties,
of which the yellow was the commonest. In the darkness of the Middle
Ages it was loosely employed for all yellow stones emanating from India,
and was finally, with increasing discernment in the characters of gem-
stones, assigned to the yellow zircon, since it was the commonest yellow
stone from India.
Considered from the scientific point of view, zircon is
by far the most interesting and the most remarkable of the
gem-stones. The problem presented by its characters and
constitution is one that still awaits a satisfactory solution.
Certain zircons, which are found as rolled pebbles in
Ceylon and never show any trace of crystalline faces, have
very nearly single refraction, and the values of the
Fig. 78.—Zircon refractive index vary from 1·790 to 1·840, and the specific
Crystal.
gravity is about 4·00 to 4·14, and the hardness is slightly
greater than that of quartz, being about 7¼. On the other hand, such stones
as the red zircons from Expailly have remarkably different properties. They
show crystalline faces with tetragonal symmetry, the faces present being
four prismatic faces mutually intersecting at right angles and four inclined
faces at each end (Fig. 78). They have large double refraction, varying from
0·044 to 0·062, which is readily discerned in a cut stone (cf. p. 41), and the
refractive indices are high, the ordinary index varying from 1·923 to 1·931
and the extraordinary from 1·967 to 1·993. Since the ordinary is less than
the extraordinary index the sign of the double refraction is positive. The
specific gravity likewise is much higher, varying from 4·67 to 4·71. The
second type, therefore, sinks in molten silver-thallium nitrate, whereas the
first type floats. The second type is also slightly harder, being about 7½ on
Mohs’s scale. By heating either of these types the physical characters are
not much altered, except that the colour is weakened or entirely driven off
and some change takes place in the double refraction. But between these
two types may be found zircons upon which the effect of heating is striking.
They seem to contract in size so that the specific gravity increases as much
as three units in the first place of decimals, and a corresponding increase
takes place in the refractive indices, and in the amount of double refraction.

The cause of these changes remains a matter of speculation. Evidently a
third type of zircon exists which is capable of most intimate association
with either of the other types, and which is very susceptible to the effect of
heat. It may be noted that stones of the intermediate type are usually
characterized by a banded or zonal structure suggesting a want of
homogeneity. The theory has been advanced that zircon contains an
unknown element which has not yet been separated from zirconium. Zircon
of the first type favours green, sky-blue, and golden-yellow colours; honey-
yellow, light green, blue, and red colours characterize the second type; and
the intermediate stones are mostly yellowish green, cloudy blue, and green.
It is another peculiarity of zircon that it sometimes shows in the
spectroscope absorption bands (p. 61), which were observed in 1866 by
Church. Many zircons do not exhibit the bands at all, and others only
display the two prominent bands in the red end of the spectrum.
Of all the gem-stones zircon alone approaches diamond in brilliance of
lustre, and it also possesses considerable ‘fire’; it can, of course, be readily
distinguished by its inferior hardness, but a judgment based merely on
inspection by eye might easily be erroneous.
According to Church, who has made a lifelong study of zircon, the green
and yellowish stones of the first variety emit a brilliant orange light when
being ground on a copper wheel charged with diamond dust, and the golden
stones of the intermediate type glow with a fine orange incandescence in
the flame of a bunsen burner; the latter phenomenon is supposed to be due
to the presence of thoria.
The leaf-green stones almost invariably show a series of parallel bands in
the interior.
Zircons vary from 5s. to 15s. a carat, but exceptional stones may be
worth more.
By far the finest stones come from Ceylon. The colourless stones are
there known as ‘Matura diamonds,’ and the hyacinth includes garnet
(hessonite) of similar colour, which is found with it in the same gravels. The
stones are always water-worn. Small hyacinths and deep-red stones come
from Expailly, Auvergne, France, and yellowish-red crystals are found in
the Ilmen Mountains, Orenburg, Russia. Remarkably fine red stones have

been discovered at Mudgee, New South Wales, and yellowish-brown stones
accompany diamond at the Kimberley mines, South Africa.

CHAP T E R XXVI I
CHRYSOBERYL
(Chrysolite, Cat’s-Eye, Cymophane, Alexandrite)

C HRYSOBERYL has at times enjoyed fleeting popularity on account of
the excellent cat’s-eyes cut from the fibrous stones, and in the form of
alexandrite it meets with a steadier, if still limited, demand. It is a gem-
stone that is seldom met with in ordinary jewellery, although its
considerable hardness befits it for all such purposes.
Chrysoberyl is in composition an aluminate of beryllium corresponding
to the formula BeAl2O4, and is therefore closely akin to spinel. It usually
contains some ferric and chromic oxides in place of alumina, and ferrous
oxide in place of beryllia, and it is to these accessory constituents that its
tints are due. Other gem-stones containing the uncommon element
beryllium are phenakite and beryl. Pale yellowish green, the commonest
colour, is supposed to be caused by ferrous oxide; such stones are known to
jewellers as chrysolite (Plate XXVII, Fig. 12). Cat’s-eyes (Plate XXIX, Fig.
1) have often also a brownish shade of green. The bluish green and dark
olive-green stones known as alexandrite (Plate XXVII, Figs. 11, 13) differ
in appearance so markedly from their fairer sisters that their common
parentage seems almost incredible. The dull fires that glow within them,
and the curious change that comes over them at night, add a touch of
mystery to these dark stones. Chromic oxide is held responsible for their
colour. The cat’s-eyes are, of course, always cut en cabochon, but otherwise
chrysoberyl is faceted.
The name of the species is composed of two Greek words, χρυσός,
golden, and βήρυλλος, beryl, and etymologically more correctly defines the
lighter-coloured stones, which were, indeed, at one time the only kind
known. Chrysolite from χρυσὁς, golden, and λίθος, stone, has much the
same significance. This name is preferred by jewellers, but in science it is
applied to an entirely different species, which is known in jewellery as
peridot. Cymophane, from κῦμα, wave, and φαίνειν, appear, refers to the

peculiar opalescence characteristic of cat’s-eyes; it is sometimes used to
designate these stones, but does not find a place within the vocabulary of
jewellery. Alexandrite is named after Alexander II, Czar of Russia, because
it first came to light on his birthday. That circumstance, coupled with its
display of the national colours, green and red, and its at one time restriction
to the mining district near Ekaterinburg, renders it dear to the heart of all
loyal Russians.
Chrysoberyl crystallizes in the orthorhombic system, and occurs in rather
dull, complex crystals, which are sometimes so remarkably twinned,
especially in the variety called alexandrite, as to simulate hexagonal
crystals. In keeping with the crystalline symmetry it is doubly refractive and
biaxial, having two directions of single refraction. The least and the greatest
of the principal indices of refraction may have any values between 1·742
and 1·749, and 1·750 and 1·757, respectively, the maximum amount of
double refraction remaining always the same, namely, 0·009. The mean
principal refractive index is close to the least; the sign of the double
refraction is therefore positive, and the shadow-edge corresponding to the
lower index, as seen in the refractometer, has little, if any, perceptible
motion when the stone is rotated. The converse is the case with corundum;
the sign is negative, and it is the shadow-edge corresponding to the greater
refractive index that remains unaltered in position on rotation of the stone.
This test would suffice to separate chrysoberyl from yellow corundum, even
if the refractive indices of the former were not sensibly lower than those of
the latter. Also, the dichroism of chrysolite is stronger than that of yellow
sapphires. In alexandrite this phenomenon is most prominent; the
absorptive tints, columbine-red, orange, and emerald-green, corresponding
to the three principal optical directions, are in striking contrast, and the first
differs so much from the intrinsic colour of the stone as to be obvious to the
unaided eye, and is the cause of the red tints visible in a cut stone. The
curious change in colour of alexandrite, from leaf-green to raspberry-red,
that takes place when the stone is seen by artificial light, is due to a
different cause, as has been pointed out above (p. 54). The effect is
illustrated by Figs. 11, 13 on Plate XXVII, which represent a fine Ceylon
stone as seen by daylight and artificial light; the influence of dichroism may
be noticed in the former picture. The specific gravity of chrysoberyl varies
from 3·68 to 3·78. In hardness this species ranks above spinel and comes
next to corundum, being given the symbol 8½ on Mohs’s scale. Certain

stones contain a multitude of microscopic channels arranged in parallel
position. When the stones are cut with their rounded surface parallel to the
channels, a broadish band of light is visible running across the stone at right
angles to them, and suggests the pupil of a cat’s eye, whence the common
name for the stones. The fact that the channels are hollow causes an
opalescence, which is absent from the quartz cat’s-eye.
The most important locality for the yellowish chrysoberyl is the rich
district of Minas Novas, Minas Geraes, Brazil, where it occurs in the form
of pebbles, and excellent material is also supplied by Ceylon, in both
crystals and rounded pebbles. Other places for chrysolite are Haddam,
Connecticut, and Greenfield, Saratoga County, New York, in the United
States, and recently in the gem-gravels near the Somabula Forest, Rhodesia.
Ceylon supplies some of the best cat’s-eyes. Alexandrite was first
discovered, as already stated, at the emerald mines near Ekaterinburg, in the
Urals; but the supply is now nearly exhausted. A poorer quality comes from
Takowaja, also in the Urals. Good alexandrite has come to light in Ceylon,
and most of the stones that are placed on the market at the present day have
emanated from that island. The Ceylon stones reach a considerable size,
often as much as from 10 to 20 carats in weight; the Russian stones have a
better colour and are more beautiful, but they are less transparent, and
rarely exceed a carat in weight. Good chrysolite may be worth from 10s. to
£2 a carat, and cat’s-eye runs from £1 to £4 a carat, depending upon the
quality. Alexandrites meet with a steady demand in Russia, and fine stones
are scarce; flawless stones about a carat in weight are worth as much as £30
a carat, and even quite ordinary stones fetch £4 a carat.
From Ceylon, that interesting home of gems, have originated some
magnificent chrysoberyls, including a superb chrysolite, 80¾ carats in
weight, and another, a splendid brownish yellow in colour and very even in
tint, and two large alexandrites, green in daylight and a rich red by night,
weighing 63⅜ and 2823/32 carats. The finest cut chrysolite existing is
probably the one exhibited in the Mineral Gallery of the British Museum
(Natural History). Absolutely flawless and weighing 43¾ carats, it was
formerly contained in the famous Hope collection, and is described on page
56 and figured on Plate XXI of the catalogue prepared by B. Hertz, which
was published in 1839; the weight there given includes the brilliants and the
ring in which it was mounted. It is shown, about actual size, in Plate

XXVII, Fig. 12. A magnificent cat’s-eye, 35·5 by 35 mm. in size, which
also formed part of the Hope collection, was included in the crown jewels
taken from the King of Kandy in 1815. The crystalline markings in the cut
stone are so arranged that the lower half shows an altar overhung by a torch.
The stone has been famous in Ceylon for many ages. It was set in gold with
rubies cut en cabochon. Two fine Ceylon alexandrites of exceptional merit,
weighing 42 and 26¾ carats, are also exhibited in the Mineral Gallery of the
British Museum (Natural History). The former is illustrated in Plate XXVII,
Figs. 11, 13, as seen in daylight and in artificial light.

CHAP T E R XXVI I I
QUARTZ
(Rock-Crystal, Amethyst, Citrine, Cairngorm, Cat’s-Eye, Tiger’s-
Eye)

A LTHOUGH the commonest and, in its natural form, the most easily
recognizable of mineral substances, quartz nevertheless holds a not
inconspicuous position among gem-stones, because, as amethyst (Plate
XXVII, Fig. 7), it provides stones of the finest violet colour; moreover, the
yellow quartz (Plate XXVII, Fig. 5) so ably vies with the true topaz that it is
universally known to jewellers by the name of the latter species, and is too
often confounded with it, and the lustrous, limpid rock-crystal even aspires
to the local title of ‘diamond.’ For all purposes where a violet or yellow
stone is required, quartz is admirably suited; it is hard and durable, and it
has the merit, or possibly to some minds the drawback, of being moderate
in price. Despite its comparative lack of ‘fire,’ rock-crystal might replace
paste in rings and buckles with considerable advantage from the point of
view of durability. The chatoyant quartz, especially in the form known as
tiger’s-eye, will for beauty bear comparison with the true cat’s-eye, which is
a variety of chrysoberyl. Except that cat’s-eye is cut en cabochon, quartz is
step- or sometimes brilliant-cut.
Ranking with corundum next to diamond as the simplest in composition
of the gem-stones, quartz is the crystallized form of silica, oxide of silicon,
corresponding to the formula SiO2. When pure, it is entirely devoid of the
faintest trace of colour and absolutely water-clear. Such stones are called
rock-crystal, and it is easy to understand why in early days it was supposed
to represent a form of petrified water. It is these brilliant, transparent stones
that are, when small, known in many localities as ‘diamonds.’ Before the
manufacture of glass was discovered and brought to perfection, rock-crystal
was in considerable use for fashioning into cups, vases, and so forth. The
beautiful tints characterizing quartz are due to the usual metallic oxides. To
manganese is given the credit of the superb purple or violet colour of

amethyst, which varies considerably in depth. Jewellers are inclined to
distinguish the deep-coloured stones with the prefix ‘oriental,’ but the
practice is to be deprecated, since it might lead to confusion with the true
oriental amethyst, which is a purple sapphire, one of the rarest varieties of
corundum. Quartz of a yellow hue is properly called citrine, but, as already
stated, jewellers habitually prefer the name ‘topaz’ for it, and distinguish the
true topaz by the prefix Brazilian—not a very happy term, since both the
yellow topaz and the yellow quartz occur plentifully in Brazil. Sometimes
the yellow quartz is termed occidental, Spanish, or false topaz. Stones with
a brownish or smoky tinge of yellow are called cairngorm, or Scotch topaz.
The colour of the yellow stones is doubtless due to a trace of ferric oxide.
Stones of a smoky brown colour are known as smoky-quartz. Rose-quartz,
which is rose-red or pink in colour and hazy in texture, is comparatively
rare; strange to say, it has never been found in distinct crystals. The tint,
which may be due to titanium, is fugitive, and fades on exposure to strong
sunlight. In milky quartz, as the name suggests, the interior is so hazy as to
impart to the stone a milky appearance. It has frequently happened that
quartz has crystallized after the formation of other minerals, with the result
that the latter are found inside it. Prase, or mother-of-emerald, which at one
time was supposed to be the mother-rock of emerald, is a quartz coloured
leek-green by actinolite fibres in the interior. Specimens containing hair-like
fibres of rutile—the so-called flêches d’amour—are common in mineral
collections, and are sometimes to be seen worked. When enclosing a
massive, light-coloured, fibrous mineral, the stones have a chatoyant effect,
and display, when suitably cut, a fine cat’s-eye effect; in tiger’s-eye the
enclosed mineral is crocidolite, an asbestos, the original blue hue of which
has been changed to a fine golden-brown by oxidation. Quartz which
contains scales of mica, hematite, or other flaky mineral has a vivid
spangled appearance, and is known as aventurine; it has occasionally been
employed for brooches or similar articles of jewellery. Rainbow-quartz, or
iris, is a quartz which contains cracks, the chromatic effect being the result
of the interference of light reflected from them; it has been artificially
produced by heating the stone and suddenly cooling it.
The name of the species is an old German mining term of unknown
meaning which has been in general use in all languages since the sixteenth
century. Amethyst is derived from ἀμέθυστος, not drunken, possibly from a
foolish notion that the wearer was exempt from the usual consequences of

unrestrained libations. Pliny suggests as an alternative explanation that its
colour approximates to, but does not quite reach, that of wine. Aventurine,
from aventura, an accident, was first applied to glass spangled with copper,
the effect being said to have been accidentally discovered owing to a
number of copper filings falling into a pot of molten glass in a Venetian
factory.
Quartz belongs to the hexagonal system of crystalline
symmetry, and crystallizes in the familiar six-sided prisms
terminated by six inclined, often triangular, faces (Fig. 79);
twins are common, though they are not always obvious
from the outward development. In accordance with the
symmetry the refraction is double, and there is one
direction of single refraction, namely, that parallel to the
edge of the prism. The ordinary refractive index has the Fig. 79.—Quartz
value 1·544, and the extraordinary 1·553, and since the Crystal.
latter is the greater, the sign of the double refraction is
positive. The double refraction is small in amount, but is large enough to
enable the apparent doubling of certain of the opposite edges of a faceted
stone to be perceptible when viewed with a lens through the table-facet. The
dichroism of the deep-coloured stones is quite distinct. Quartz has only
about the same amount of colour dispersion as ordinary glass, and lacks,
therefore, ‘fire.’ The application of strong heat tends, as usual, to weaken or
drive off the colour. Thus the dense smoky-quartz found in Spain, Brazil,
and elsewhere is converted into stones of a colour varying from light yellow
to reddish brown according to the amount and duration of the application.
In the case of amethyst the colour is changed to a deep orange, or entirely
driven off if the temperature be high enough. Its density is very constant,
varying only from 2·654 to 2·660; the purest stones are the lightest. To it
has been assigned the symbol 7 on Mohs’s scale of hardness.
To physicists quartz is one of the most interesting of minerals because of
its power of rotating, to an extent depending upon the thickness of the
section, the plane of polarization of a beam of light traversing it in a
direction parallel to the prism edge. It appears, moreover, from a study of
the pyro-electric and general physical characters, that its molecular
structure has a helical arrangement, which, like all screws, may have a
right- or left-handed character. Amethyst is, in fact, invariably composed of

separate twin individuals, alternately right- and left-handed; in some
remarkable crystals the section at right angles to the prism edge is
composed of triangular sectors, alternately of different hands and of
different tints—purple and white. To the twinning is due the rippled fracture
and the feathery inclusions so characteristic of amethyst.
Besides its use for ornamental purposes, quartz finds a place as the
material for lenses intended for delicate photographic work, because its
transparency to the ultra-violet light is so much greater than that of glass.
Spectacle lenses made of it are in demand, because they are not liable to
scratches, and retain, therefore, their polish indefinitely. When fused in the
oxyhydrogen flame, quartz becomes a silica glass, of specific gravity 2·2
and hardness 5 on Mohs’s scale, which has proved of great service for
laboratory ware, because it withstands sudden and unequal heating without
any danger of fracture; it has also in fine threads been invaluable for
delicate torsion work, because it acquires not the smallest amount of
permanent twist, in this respect being superior to the finest silk threads.
Clear rock-crystal fetches little more than the cost of the cutting; citrine
and amethyst are worth from 1s. to 5s. a carat, depending upon the quality
and size of the stone; smoky-quartz is practically valueless; rose-quartz
realizes less than 1s. a carat; and the value of cat’s-eye is also small—only
1s. to 2s. 6d. a carat. Tiger’s-eye at one time commanded as much as 25s. a
carat, but the supply exceeded the demand, with the consequent collapse in
the price.
Beautiful, brilliant, and limpid rock-crystal is found in various parts of
the world: in the Swiss Alps, at Bourg d’Oisans in the Dauphiné Alps,
France, in the famous Carrara marble, in the Marmaros Comitat of
Hungary, and in the United States, Brazil, Madagascar, and Japan. Small
lustrous stones, known in their localities as ‘Isle of Wight,’ ‘Cornish,’ or
‘Bristol diamonds,’ are found in our own country. Brazil supplies stones out
of which have been cut the clear balls used in crystal-gazing. The finest
amethysts come from Brazil—especially the State of Rio Grande do Sul—
and from Uruguay, India, and the gem-gravels of Ceylon; good stones also
occur at Ekaterinburg, in the Ural Mountains. A splendid Brazilian
amethyst, weighing 334 carats, and two Russian stones—one hexagonal in
contour, weighing 88 carats, and the other, a deep purple in colour with a
circular table, weighing 73 carats—are exhibited in the British Museum

(Natural History). Cairngorm is known from the place of that name in
Banffshire, Scotland, whence fine specimens have emanated; it is a gem
much valued in that country. Fine cairngorm has also originated from Pike’s
Peak, Colorado. Splendid yellow stones have had their birth in the States of
Minas Geraes, São Paulo, and Goyãz, of Brazil—especially in the last. The
fine Spanish smoky-quartz, which, as already stated, turns yellow on
heating, comes from Hinojosa, in the Province of Cordova. The delicate
rose-quartz is known at Bodenmais in Bavaria, Paris in Maine, United
States, and Ekaterinburg in the Ural Mountains. The finest cat’s-eyes are
found in India and Ceylon, and are high in favour with the natives. Greenish
stones of an inferior quality are brought from the Fichtelgebirge in Bavaria,
and are sold as ‘Hungarian cat’s-eyes,’ despite the fact that no such stone
occurs in Hungary—another instance of jewellers’ disdain for accuracy.
Tiger’s-eye occurs in considerable quantity in the neighbourhood of
Griquatown, Griqualand West, South Africa. A silicified crocidolite, in
which the blue colour is retained, comes also from Salzburg, and is known
as sapphire- or azure-quartz, or siderite.
Certain of the pebbles found on the seashore of our coasts, especially off
the Isle of Wight and North Wales, cut into attractive, clear stones, more or
less yellow in colour; but examples suitable for the purpose are not so
numerous as might be supposed, and do not reward any casual search. Les
affaires sont les affaires. The local lapidary, instead of explaining that the
pebbles brought to him are not worth cutting, finds it more convenient and
profitable to substitute for them other, inferior and badly cut, stones, bought
by the gross, or even paste stones; the customer, on the other hand, is
contented with a pretty bauble, and is not grateful for the information that it
might have been obtained for a fraction of the sum paid.

CHAP T E R XXI X

CHALCEDONY, AGATE, ETC.

C HALCEDONY and agate, and their endless varieties, are composed
mainly of silica, but the separate individual crystals are so small as to
be invisible to the unaided eyesight, and occasionally are so extremely
minute that the structure is almost amorphous. The colour and appearance
vary greatly, depending upon the impurities contained in the stone, and,
since both have been made a criterion for differentiation of types, a host of
names have come into use, none of which are susceptible of strict
definition. On the whole, these stones may be divided into two groups:
chalcedony, in which the structure is concretionary and the colour
comparatively uniform, and agate, in which the arrangement takes the form
of bands, varying greatly in tint and colour.
The refraction, though double in the individual, is irregular over the stone
as a whole, and the indices approximate to 1·550. The specific gravity
ranges from 2·62 to 2·64, depending upon the impurities present. The
degree of hardness is about the same as that of quartz, namely, 7 on Mohs’s
scale. All kinds are more or less porous, and stones of a dull colour are
therefore artificially tinted after being worked.
The term chalcedony, derived from χαλκηδών the name of a town in Asia
Minor, is usually confined to stones of a greyish tinge. Stones artificially
coloured an emerald green have been cut and put upon the market as
‘emeraldine.’ Carnelian is a clear red chalcedony, and sard is somewhat
similar, but brownish in tint. Chrysoprase is apple-green in colour, nickel
oxide being supposed to be the agent. Prase (cf. p. 240), which is a dull
leek-green in hue, may also in part be referred here; the name comes from
πράσμον, a leek. Plasma, which may have the same derivation, is a brighter
leek-green. Jasper is a chalcedony coloured blood-red by iron oxide, while
bloodstone is a green chalcedony spotted with jasper; they are popular
stones for signet rings. Flint, an opaque chalcedony, breaks with a sharp
cutting edge, and was much in request with early man as a tool or a
weapon; its property of giving sparks when struck with steel rendered it

invaluable before the invention of matches. Hornstone is somewhat similar,
but more brittle, while chert is a flinty rock.
Agate, named after the river Achates in Sicily, where it was found at the
time of Theophrastus, has a peculiar banded structure, the bands being
usually irregular in shape, following the configuration of the cavity in
which it was formed. Moss-agate, or mocha-stone, contains moss-like
inclusions of some fibrous mineral. Onyx is an agate with regular bands, the
layers having sharply different colours; when black and white, it has, in
days gone by, been employed for cameos. Sardonyx is similar in structure,
but red and white in colour. Agate is used in delicate balances for
supporting the steel knife-edges of the balance itself and of the panholders,
and is largely employed—especially when artificially coloured—for
umbrella handles and similar articles.
Chalcedony and agate are found the whole world over, but India, and
particularly Brazil, are noted for their fine carnelians and agates.

CHAP T E R XXX

OPAL
(White Opal, Black Opal, Fire-Opal)

T HAT opal in early times excited keen admiration is evident from Pliny’s
enthusiastic description of these stones: “For in them you shall see the
burning fire of the carbuncle, the glorious purple of the amethyst, the green
sea of the emerald, all glittering together in an incredible mixture of light.”
During much of last century, owing to the foolish superstition that ill-luck
dogs the footsteps of the wearer, the species lay under a cloud, which has
even now not quite dispersed, but exercises a prejudicial effect upon the
fortunes of the stone. It has, however, recently attracted considerable
attention owing to the discovery of the splendid black opals in Australia; at
one moment black with the darkness of night, at the next by a chance
movement glowing with vivid crimson flame, such stones may justly be
considered the most remarkable in modern jewellery. At the present day
opal is divided by jewellers roughly into two main groups: ‘white’ (Plate
XXVII, Fig. 6) and ‘black’ (Plate XXVII, Fig. 9), according as the tint is
light or dark, fire-opal (Plate XXVII, Fig. 10) standing in a separate
category.
Opal differs from the rest of the principal gem-stones in being not a
crystalline body, but a solidified jelly, and it depends for its attractiveness
upon the characteristic play of colour, known, in consequence, as
opalescence (cf. p. 39), which arises from a peculiarity in the structure.
Opal is mainly silica, SiO2, in composition, but contains in addition an
amount of water varying in precious opal from 6 to 10 per cent. As the
original jelly cooled, it became riddled throughout with cracks, which were
afterwards generally filled with opal matter, containing a different amount
of water, and therefore differing slightly in refractivity from the original
substance. The structure not being quite homogeneous, each crack has the
same action upon light as a soap-film, and gives rise to precisely similar
phenomena; the thinner and more uniform the cracks, the greater the

splendour of the chromatic display, the particular tint depending upon the
direction in which the stone is viewed. The cracks in certain opals were not
filled up, and therefore contain air. Such stones appear opaque and devoid
of opalescence until plunged into water; they are consequently known as
hydrophane, from ὕδωρ, water, and φαίνεσθαι, to make appear. Owing to
the effect of total-reflection, light was stopped on the hither side of the
cracks before they were filled with water, which is not far inferior to opal in
refractivity; it is surprising how much water these stones will absorb.
Opal is colourless when pure, but is nearly always more or less milky and
opaque, or tinted various dull shades by ferric oxide, magnesia or, alumina.
The so-called black opal is generally a dark grey or blue, and very rarely
quite black. That the coloration is not due to ordinary absorption, but to the
action of cracks in the stone, is shown by the fact that the transmitted light
is complementary to the reflected light; the blue opal is, for instance, a
yellow when held up so that light has passed through it. In many black
opals the opalescent material occurs in far too tiny pieces to be cut
separately, and the whole iron-stained matrix is cut and polished and sold
under the name ‘opal-matrix.’ The reddish and orange-coloured stones
known as fire-opal have pronounced colour and only slight milkiness; they
display the customary opalescence in certain directions. These stones are
often faceted, but otherwise opals are cut en cabochon, either flat or steep—
generally the former in brooches and pendants, and the latter in rings. Opal
is somewhat soft, varying from 5 to 6½ on Mohs’s scale, and is therefore
easily scratched. The specific gravity ranges from 2·10 to 2·20, and the
refractive index from 1·444 to 1·464, the refraction, of course, being always
single. It is unwise to immerse opals in liquids on account of their porosity.
The name opal comes to us through the Latin opallus, which was used
for the same species as understood by the term at the present day, but the
word has a far older origin, which has not been traced. The Romans also
called the mineral pæderos, the Greek form of Cupid, a name applied to all
rosy stones. The name cacholong, for the bluish-white porcelain variety,
which is very porous and adheres to the tongue, is of Tartar origin; the stone
is highly valued in the East.
The oldest mines, which up to quite a recent date were the only extensive
deposit of opal known, were at Cserwenitsa, near Kashau, in Hungary.
From them in all probability emanated the opals known to the Romans. The

opals from this locality were generally quite small, and large pieces were
rare and commanded high prices. The Hungary mines, however, proved
quite unable to compete with the rich fields at White Cliffs, New South
Wales, in spite of the efforts that were made to depreciate and exclude from
the market the new stones, and at the present time few of the opals on the
market come from them. As so often happens, the White Cliffs deposit was
discovered by accident. In 1889 a hunter, when tracking a wounded
kangaroo, chanced to pick up an attractively coloured opal. The district is so
waterless and forbidding that, but for such a chance, the opals might have
long lain hidden. They occur in seams in deposits of Cretaceous Age in a
variety of ways, filling cavities in rocks or sandstones, or cracks in wood, or
replacing wood, saurian bones, and some spiky mineral, which may have
been glauberite. In recent years, another rich deposit was discovered farther
north, on both sides of the boundary between Queensland and New South
Wales. The field is remarkable for the darkness of its opals, which are called
‘black opal’ in contradistinction to the lighter-coloured stones previously
known. From Lightning Ridge in New South Wales come stones stained
deep black which quite merit the designation black opal. The sandstone in
which they are found is rich in iron, and this is no doubt responsible for the
deepness of their tint. Mexico is noted for the fire-opal, which is found at
Esperanza, Queretaro, and Zimapan; but other kinds of opal also are found
at these places.

PLATE XXVIII

OPAL MINES, WHITE CLIFFS, NEW SOUTH WALES

The price of opal varies greatly, according to the intrinsic colour and the
uniformity and brilliance of the opalescence. Common opal can be bought
at as low a rate as 1s. a carat, while black opal ranges from 10s. to £8 a
carat; but a good dark stone displaying a flaming opalescence commands a
fancy figure, fine stones of this class being exceedingly rare. Fire-opal
enjoys only a limited popularity now, though a few years ago it was in some
demand; the price runs from 2s. to 10s. a carat.

CHAP T E R XXXI

FELSPAR
(Moonstone, Sunstone, Labradorite, Amazon-Stone)

T HOUGH second to none among minerals in scientific interest, whether
regarded from the point of view of their crystalline characters or the
important part they play in the formation of rocks, the group included under
the general name felspar occupies but a humble place in jewellery. It
consists of three distinct species, orthoclase, albite, and anorthite, which are
silicates of aluminium, and potassium, sodium, or calcium, corresponding
to the formulae KAlSi3O8, NaAlSi3O8, and CaAl2Si2O8 respectively, and
also of species intermediate in composition between albite and orthoclase,
or albite and anorthite. While differing in crystalline symmetry, all are
characterized by two directions of cleavage which are nearly at right angles
to one another. The double refraction, which is slight in amount, is biaxial
in character and variable in sign. The values of the least and greatest of the
indices of refraction range between 1·52 and 1·53, and 1·53 and 1·55
respectively, the double refraction at the same time varying from 0·007 to
0·012. The specific gravity lies between 2·48 and 2·66, and the hardness
ranges between the degrees 6 and 7 on Mohs’s scale.
Moonstone (Plate XXIX, Fig. 4), which is mainly pure orthoclase, alone
is at all common in jewellery. It forms such an admirable contrasting frame
for large coloured stones that it deserves greater popularity; no doubt the
cheapness of the stones militates against their proper appreciation. The
milky, bluish opalescence from which they take their name is caused by the
reflection of light at the thin twin-lamellæ of which the structure is
composed. They are always cut more or less steeply en cabochon. The
finest stones were at one time cut from the felspar that came from the St.
Gothard district in Switzerland and was in consequence known as adularia
from the neighbouring Adular Mountains, somewhat incorrectly, since none
occurs at the latter locality. At the present day practically all the moonstones

on the market come from Ceylon. They run in price from £3 to £20 per oz.
(28 grams).
Sunstone is a felspar containing flakes of hematite or goethite which
impart a spangled bronze appearance to the stones. Good material occurs in
parts of Norway. The remarkable sheen of labradorite or blue felspar has its
origin in the interference of light at lamellar surfaces in the interior; the
uniformity of the colour over comparatively large areas testifies to the
regularity of the lamellar arrangement. The finest specimens were brought
from the Isle of St. Paul off the coast of Labrador, where they were first
discovered in 1770; large masses also occur on the coast itself. Amazon-
stone is an opaque green felspar which occurs in the Ilmen Mountains,
Orenburg, Russia, and at Pike’s Peak, Colorado, United States. It obtains its
name from the Amazon River, where, however, none has ever been found;
there may have been some confusion with a jade or similar stone.
Occasionally clear colourless felspar has been faceted, and then closely
resembles rock-crystal. A careful determination of the refractive indices and
the specific gravity serves to discriminate between them.

PLATE XXIX

1. CAT’S EYE 2. PERIDOT 3. PERIDOT 4. MOONSTONE

5. HESSONITE 6. PYROPE 8. ALMANDINE
7. DEMANTOID

9. SPODUMENE 11. HIDDENITE
10. KUNZITE

12. ZIRCON 14. ZIRCON 15. ANDALUSITE
13. ZIRCON

17. TURQUOISE
16. NEPHRITE 18. JADEITE
GEM-STONES

CHAP T E R XXXI I

TURQUOISE, ODONTOLITE, VARISCITE

O F all the opaque stones turquoise (Plate XXIX, Fig. 17) alone finds a
prominent place in jewellery and can aspire to rank with the precious
stones. The colour varies from a sky-blue or a greenish blue to a yellowish
green or apple-green. Only the former tints, which are at the same time the
rarer, are in general demand, and they possess the great advantage of
harmonizing with the tint of the gold setting. The blue colours are,
especially in the case of the Siberian stones, by no means permanent, and
fade in course of time. Turquoise is amorphous and seldom crystalline, and
is therefore somewhat porous; it should consequently never be immersed in
liquids or be contaminated with greasy and dirty matter lest the dreaded
change of colour be brought about. The stones are translucent in thin
sections, and a good observation is possible with the refractometer if the
back of the stone is flat and polished, since only the section immediately
adjacent to the instrument is concerned; the refractive index is about 1·61.
The specific gravity varies from 2·75 to 2·89. Turquoise has a hardness of
slightly under 6 on Mohs’s scale, and takes a good polish, which is fairly
durable, since on account of the comparative opacity of the stones scratches
on the surface are not very noticeable. In composition it is a complex
phosphate of aluminium and copper, corresponding to the formula CuOH.
[6Al(OH)2].H5.(PO4)4, with ferric oxide replacing some alumina. The blue
colour is due to the copper constituent, and the predominance of iron may
cause the greenish shades; but the water contained in the stones plays no
mean part, since they turn a dirty green when it is driven off. The faded
colour can sometimes be restored by immersion of the stone in ammonia
and subsequent application of grease, but the effect is not lasting. Attempts
are sometimes made to improve inferior stones by impregnating them with
Berlin blue, but with only qualified success. Turquoises are said to be
affected by the perspiration from the skin.
The name of the species comes from a French word meaning Turkish,
and arises from the fact that the gem-stone first reached Europe by way of

Turkey. Another, but less obvious, suggestion is that it is derived from the
Persian name for the species, piruzeh. Our turquoise and other phosphates
of similar appearance were probably known to Pliny under the three names
callais, callaina, and callaica.
The finest turquoise still comes from the famous mines near Nishapur in
the Persian province of Khorassan, where it was known in very ancient
times; it is found with limonite filling the cracks and cavities in a brecciated
porphyritic trachyte. Pieces of the turquoise and limonite from here are
sometimes cut without removal of the latter, and sold as ‘turquoise-matrix,’
when the precious stones are too tiny to be worth separate working. It also
occurs at Serbâl in the Sinai Peninsula. Among the more recent localities
may be mentioned Los Cerillos Mountains, New Mexico; Sierra Nevada,
Nevada, where pale blue and green stones are found; San Bernardino
County, California, where again the stones are rather pale; and Arizona,
where it occurs in pale greenish-blue stones.
Some of the stones that have been seen are not the true turquoise but
odontolite, or bone turquoise, which consists of the teeth and bones of
mastodon or other extinct animals, phosphate of iron being the colouring
material. These stones may easily be recognized by their organic structure,
which is clearly visible if viewed with a strong lens or under the
microscope. Moreover, odontolite invariably contains some calcium
carbonate, and effervescence takes place if it be touched with hydrochloric
acid. Turquoise dissolves in hydrochloric acid, but without effervescence,
and since it contains copper, a fine blue colour is imparted to the solution
by the addition of ammonia. Odontolite has a higher specific gravity, 3·0 to
3·5, but lower hardness, 5 on Mohs’s scale.
Variscite, the hydrated phosphate of aluminium, corresponding to the
formula AlPO4 + 2H2O, is found in masses resembling a greenish turquoise,
but it is much softer, being only 4 on Mohs’s scale. The specific gravity is
2·55. Round nodular masses of variscite are found in Utah.

CHAP T E R XXXI I I

JADE

T HOUGH not usually accounted precious among European nations or in
Western civilization in general, jade was held in extraordinary esteem
by primitive man, and was fashioned by him into ornaments and utensils,
often of considerable beauty, and even at the present day it ranks among the
Chinese and Japanese peoples above all precious stones; indeed, the
Chinese word Yu and the Japanese words Giyuku or Tama signify both jade
and precious stones in general. According to the Chinese, jade is the
prototype of all gems, and unites in itself the five cardinal virtues—Jin,
charity; Gi, modesty; Yu, courage; Ketsu, justice; and Chi, wisdom. When
powdered and mixed with water, it is supposed to be a powerful remedy for
all kinds of internal disorders, to strengthen the frame and prevent fatigue,
to prolong life, and, if taken in sufficient quantity just before death, to
prevent decomposition.
Jade is a general term that includes properly two distinct mineral species,
nephrite or greenstone, and jadeite, which are very similar in appearance,
both being fibrous and tough in texture, and more or less greenish in colour;
but it is also applied to other species such as saussurite, californite,
bowenite, and plasma, which have somewhat similar characters. The word
jade is a corruption of the Spanish pietra di hijada, kidney-stone, in allusion
to its supposed efficacy in diseases of that organ.
Nephrite or greenstone (Plate XXIX, Fig. 16) is the commoner of the two
jades. It is closely allied to the mineral hornblende, a silicate of magnesium,
iron, and calcium corresponding to the formula Ca(Mg,Fe)3(SiO3)4, the
magnesia being replaceable by ferrous oxide. Microscopic examination
shows that the structure consists of innumerable independent fibres foliated
or matted together, the former character giving rise to a slaty and the latter
to a horny appearance in the stone as seen by the unaided eye. The colour
varies from grey to leaf- and dark-green, the tint deepening as the relative
amount of iron in the composition increases, and brown tints result from the
oxidation of the iron along cracks in the stone. The hardness is 6½ on

Mohs’s scale; nephrite is therefore about as hard as ordinary glass and
softer than quartz. When polished, it always acquires a greasy lustre. The
specific gravity ranges from 2·9 to 3·1. The least and greatest of the
principal refractive indices are 1·606 and 1·632 respectively, the double
refraction being biaxial and negative; the coloured fibres also display
dichroism. All these differential effects are, however, masked in the stone
because of the irregularity of the aggregation. Nephrite is fusible before the
blowpipe, but only with difficulty. Its name is derived from the Greek word
νεφρός, kidney, the allusion being the same as for jade.
Many of the prehistoric implements found in Mexico and in the Swiss
Lake Habitations are composed of nephrite, but it is uncertain where the
mineral was obtained. Much of the material used by the Chinese at the
present time comes from spots near the southern boundary of Eastern
Turkestan, especially in the valleys of the rivers Karakash and Yarkand in
the Kwen Lun range of mountains; it is also found farther north at the river
Kashgar. It occurs in various provinces of China, namely, Shensi, Kwei
Chau, Kwang Tung, Yunnan, and Manchuria. Gigantic waterworn boulders
have been found in the Government of Irkutsk, near Lake Baikal, in eastern
Siberia, the first discovery being made in the bed of the Onot stream by the
explorer and prospector J. P. Alibert, in 1850. A large boulder of this kind,
weighing over half a ton (1156 lb., or 524·5 kg.), is exhibited in the Mineral
Gallery of the British Museum (Natural History). An enormous mass,
weighing over 2 tons (4718 lb., or 2140 kg.), was discovered at
Jordansmühl, Silesia, by Dr. G. F. Kunz, and is now in the magnificent
collection of jade formed by Mr. Heber R. Bishop. Beautiful greenstone
occurs in New Zealand, particularly in the Middle Island. The Maoris have
long used it for various useful and ornamental purposes, the most common
being indicated by their general name for the species, punamu, axe-stone;
kawakawa is the ordinary green variety, a fine section of which is shown on
the wall of the Mineral Gallery of the British Museum (Natural History),
while inanga, a grey variety, and kahurangi, a pale-green and translucent
variety, are rare and highly prized.
Jadeite (Plate XXIX, Fig. 18) is by far the rarer of the two jades, and is
the choicest gem with the Chinese. In composition it is a silicate of sodium
and aluminium with the formula NaAl(SiO3)2, corresponding to the lithium
mineral spodumene (p. 265). It has the same toughness and greasy lustre as

nephrite, but is harder, being represented by the symbol 7 on Mohs’s scale,
and thus only slightly, if at all, softer than quartz. The other characters are
also higher; the specific gravity is about 3·34, and the least and greatest of
the principal refractive indices are 1·66 and 1·68, the double refraction
being biaxial and negative. The colour varies from white to almost an
emerald green, the latter being especially prized, and often the green colour
runs in streaks through the white. Jadeite fuses readily before the blowpipe
to blebby glass, more easily than is the case with nephrite.
The finest jadeite comes from the Mogaung district in Upper Burma,
where it is found in boulders and also with albite in dykes in a dark-green
serpentine. The export trade to China, which absorbs practically the whole
of the output, is exceedingly valuable, and realizes nearly as much as the
produce of the ruby mines. Jadeite is also found in the Shensi and Yunnan
provinces of China, and in Tibet.

A few words may be said about the other jade-like minerals. Saussurite,
which is named after H. B. de Saussure, has resulted from the
decomposition of a felspar, and is nearly akin to the mineral zoisite. It has
the customary toughness of structure, and is greenish grey to white in
colour. Its specific gravity is about 3·2, and hardness 6½ to 7 on Mohs’s
scale. It occurs near Lake Geneva. Bowenite is a green serpentine (p. 289)
which is found at Smithfield, Rhode Island, U.S.A., and in New Zealand
and Afghanistan. Californite and plasma are compact varieties of idocrase
(p. 275) and chalcedony (p. 247) respectively. Verdite is a stone of rich
green colour which is found in the form of large boulders in the North Kaap
River, South Africa; it is composed of green mica (fuchsite) and some
clayey matter.
Jade has of recent years been imitated in glass, but the latter is
recognizable by its vitreous lustre and inferior hardness, and sooner or later
by its frangibility.

CHAP T E R XXXI V

SPODUMENE, IOLITE, BENITOITE

Spodumene
(Kunzite, Hiddenite)

T ILL a few years ago scarcely known outside the ranks of mineralogists,
spodumene suddenly leaped into notice in 1903 upon the discovery of
the lovely lilac-coloured stones (Plate XXIX, Fig. 10) at Pala, San Diego
County, California; they shortly afterwards received the name kunzite after
the well-known expert in gems, Dr. G. F. Kunz. The stones were found here
in a pegmatite dyke, and were of all shades, ranging from pale pink to deep
lilac, and at times as much as 150 carats in weight. Paler kunzite occurs
with beryl and tourmaline at Coahuila Mountain in Riverside County,
California, and colourless stones have recently come to light in Madagascar.
Kunzite is remarkable for its wonderful dichroism; the beautiful violet tint
that springs out in one direction comes with greater surprise because of the
uninteresting yellowish tints in other directions. Unlike spodumene in
general, kunzite is phosphorescent under the influence of radium.
The emerald-green variety (Plate XXIX, Fig. 11), named hiddenite after
Mr. W. E. Hidden, who discovered in 1881 the only known occurrence, in
Alexander County, North Carolina, would no doubt have become popular
had the supply of material not been so very limited; few stones were found,
and the variety has never come to light elsewhere. The colour is supposed to
be due to chromic acid. Hiddenite being also dichroic, the tint varies with
the direction.
Spodumene is ordinarily rather a pale yellowish in hue, and, as its name
(which is derived from σποδίος, ash-coloured) suggests, is not very
attractive. Clear, lemon-yellow stones (Plate XXIX, Fig. 9) are found in
Brazil and Madagascar.

The species is interesting scientifically because it contains the rare
element lithium; it is a silicate of aluminium and lithium, corresponding to
the formula LiAl(SiO3)2. The double refraction is biaxial in character and
positive in sign, the least and greatest of the refractive indices being 1·660
and 1·675; the specific gravity is 3·185, and hardness 6½ to 7 on Mohs’s
scale. Spodumene has an easy cleavage, and the cut stones call therefore for
careful handling, lest they be flawed or fractured. Two faceted stones, a
beautiful kunzite and a fine hiddenite, weighing 60 and 2½ carats
respectively, are exhibited in the British Museum (Natural History).

Iolite
Known also by various other names—cordierite, dichroite, and water-
sapphire (saphire d’eau)—this species owes its interest to the remarkable
dichroism characterizing it, the principal colours—smoky-blue and
yellowish white—being in such contrast as to be obvious to the unaided
eye. The stones that are usually worked have intrinsically a smoky-blue
colour, and are found in waterworn masses in the river-gravels of Ceylon,
whence is the origin of the name water-sapphire. Iolite, from ἴον, violet, and
λίθος, stone, refers to the colour; cordierite is named after Cordier, a French
geologist, who first studied the crystallography of the species; and
dichroite, of course, alludes to the most prominent character of the species.
Iolite is a silicate of aluminium and of magnesium and iron
corresponding to the formula H2(Mg,Fe)4Al8Si10O37. The double refraction
is small in amount, biaxial in character, and negative in sign, the least and
greatest of the refractive indices being 1·543 and 1·551; the specific gravity
is 2·63, and hardness 7 on Mohs’s scale. Iolite, if used, is worked and
polished; it is seldom faceted. A large worked piece, weighing 177 grams,
which was formerly in the Hawkins Collection, is exhibited in the British
Museum (Natural History).

Benitoite
The babe among gem-stones, benitoite first saw the light of day a few
years ago, early in 1907. It occurs with the rare mineral neptunite, which
was previously known only from Greenland, in narrow veins of natrolite in
Diablo Range near the head-waters of the San Benito River, San Benito

County, California. Despite careful search the species has not been found
except within the original restricted area. To science it is interesting both
because of its composition, a silico-titanate of barium, corresponding to the
formula BaTiSi3O9, and because its crystals belong to a class of crystalline
symmetry which has hitherto not been represented among minerals. The
double refraction is uniaxial, and since the ordinary index of refraction is
1·757 and the extraordinary 1·804, it is positive in sign and large in amount,
namely, 0·047. The stones are characterized by strong dichroism, the colour
corresponding to the ordinary ray being white, and to the extraordinary
greenish blue to indigo depending upon the tint of the stone. To obtain the
best effect the stone must therefore be cut with the table-facet parallel to the
crystallographic axis. The specific gravity is 3·65, and hardness 6½ on
Mohs’s scale. When first discovered the species was supposed to be
sapphire, and many stones were cut and sold as such. It is, however, much
softer than sapphire, and is readily distinguished by its optical characters,
since it possesses greater double refraction and of differing sign, so that,
when tested with the refractometer, the shadow-edge corresponding to the
lower index of refraction remains fixed in the case of benitoite, whereas the
contrary happens with sapphire. Benitoite also, unlike sapphire, fuses easily
to a transparent glass. Its blue colour, which is supposed to be due to a
small amount of free titanic acid present, appears to be stable. Several
stones as large as 1½ to 2 carats in weight have been found. The largest of
all, perfectly flawless, weighs just over 7 carats, and is remarkable because
it is about three times the next largest in point of weight; it is the property of
Mr. G. Eacret, of San Francisco.

CHAP T E R XXXV

EUCLASE, PHENAKITE, BERYLLONITE

Euclase

T HIS species comes near beryl in chemical composition, being a silicate
of aluminium and beryllium corresponding to the formula
Be(AlOH)SiO4, and closely resembles aquamarine in colour and
appearance when cut. Owing to the rarity of the mineral good specimens
command high prices for museum collections, and it is seldom worth while
cutting it for jewellery. It derives its name from its easy cleavage, εὖ easily,
and κλάσις fracture. The double refraction is biaxial in character and
positive in sign, the least and greatest of the refractive indices being 1·651
and 1·670 respectively; the specific gravity is 3·07, and the hardness 7½ on
Mohs’s scale. The colour is usually a sea-green, but sometimes blue.
Euclase occurs with topaz at the rich mineral district of Minas Novas,
Minas Geraes, Brazil, and has also been found in the Ural district, Russia.

Phenakite
Another beryllium mineral, phenakite owes its name to the frequency
with which it has been mistaken for quartz, being derived from φέναξ,
deceiver. The clear, colourless crystals, somewhat complex in form, have at
times been cut, but they lack ‘fire,’ and despite their brilliant lustre meet
with little demand. The composition is a silicate of beryllium corresponding
to the formula Be2SiO4. The double refraction is uniaxial, and since the
ordinary, 1·652, is less than the extraordinary index, 1·667, it is positive in
sign; the specific gravity is 2·99, and the hardness is almost equal to that of
topaz, being about 7½ to 8 on Mohs’s scale.
Fine stones have long been known near Ekaterinburg in the Ural
Mountains, and have recently been discovered in Brazil.

Beryllonite

As its name suggests, this mineral also contains beryllium, being a soda
phosphate corresponding to the formula NaBePO4. Clear, colourless stones,
which occur at Stoneham, Maine, U.S.A., have been cut, but the lack of
‘fire,’ the easy cleavage, and comparative softness, the symbol being 5½ on
Mohs’s scale, unfit it for use in jewellery. The double refraction is biaxial in
character and negative in sign, the least and the greatest of the refractive
indices being 1·553 and 1·565 respectively.

CHAP T E R XXXVI

ENSTATITE, DIOPSIDE, KYANITE, ANDALUSITE,
IDOCRASE, EPIDOTE, SPHENE, AXINITE, PREHNITE,
APATITE, DIOPTASE

Enstatite
(‘Green Garnet’)

T HE small green stones which accompany the diamond in South Africa
have been cut and put on the market as ‘green garnet.’ They are,
however, in no way connected with garnet, but belong to a mineral species
called enstatite, which is a silicate of magnesium corresponding to the
formula MgSiO3; the green colour is due to a small amount of ferrous oxide
which replaces magnesia. The double refraction is biaxial in character and
positive in sign, the least and greatest of the refractive indices being 1·665
and 1·674 respectively; the specific gravity ranges from 3·10 to 3·13, and
the hardness is only about 5½ on Mohs’s scale. The dichroism is
perceptible, the twin-colours being yellowish and green, and, as usual, is
more pronounced the deeper the colour of the stone. There is also a good
cleavage in two different directions.
With increasing percentage amount of iron enstatite passes into
hypersthene. The colour becomes a dark brownish green, and an increase
takes place in the physical constants, the least and greatest of the refractive
indices attaining to 1·692 and 1·705 respectively, and the specific gravity
ranging from 3·4 to 3·5. Hypersthene is never sufficiently transparent for
faceting, but when spangled with small scales of brookite it is sometimes
cut en cabochon.
The name enstatite is derived from ἐνστάτης, an opponent, referring to
the infusibility of the mineral before the blowpipe, and hypersthene comes
from ὑπερσθένος, very tough.

An altered enstatite, leek-green in colour and with nearly the composition
of serpentine (p. 289), has been cut en cabochon. It has much lower specific
gravity, only 2·6, and lower hardness, 3½ to 4 on Mohs’s scale. It is named
bastite from Baste in the Harz Mountains, where it was first discovered.

Diopside
This species, which is also known as malacolite and alalite, provides
stones of a leaf-green colour which have occasionally been cut. It is a
silicate of calcium and magnesium corresponding to the formula
MgCa(SiO3)2, but usually contains in place of magnesia some ferrous
oxide, to which it owes its colour; with increase in the percentage amount
of iron the colour deepens and the physical constants change. The double
refraction is large in amount, 0·028, biaxial in character, and positive in
sign. The least and greatest of the refractive indices corresponding to the
stones suitable for jewellery range about 1·671 and 1·699 respectively, but
they may be as high as 1·732 and 1·750 in the two cases. The specific
gravity varies from 3·20 to 3·38, and the hardness from 5 to 6 on Mohs’s
scale. Dichroism is noticeable in deep-coloured stones, but is not very
marked.
The name diopside comes from δίς, double, and ὄψις, appearance, in
allusion to the effect resulting from the double refraction; malacolite is
derived from μαλακός, soft, because the mineral is softer than the felspar
associated with it; and alalite is named after the principal locality, Ala
Valley, Piedmont, Italy.

Kyanite
Kyanite, also known as disthene, is interesting for two reasons. Its
structure is so grained in character that the hardness varies in the same stone
from 5 to 7 on Mohs’s scale; it can therefore be scratched by a knife in
some directions, but not in others (p. 79). It has the same chemical
composition as andalusite, both being silicates of aluminium corresponding
to the formula Al2SiO5, but possesses very different physical characters, a
fact which shows how large a share the molecular grouping has in
determining the aspect of crystallized substances. It is biaxial with small
negative double refraction, the least and greatest of the refractive indices

being 1·72 and 1·73 respectively; the specific gravity is 3·61. It occurs in
sky-blue prismatic crystals, whitish at the edges, in schist near St. Gothard,
Switzerland. It is seldom cut.
Kyanite is derived from its colour, κύανος blue, and disthene, from its
variable hardness, δίς, twice, and σθένος, strong.

Andalusite
Andalusite bears no resemblance whatever to kyanite, although, as has
been stated above, the composition of the two species is essentially the
same. It is usually light bottle-green in colour, and more rarely brown and
reddish. Its extreme dichroism is its most remarkable character, the twin
colours being olive-green and red. The reddish gleams that are reflected
from the interior are in sharp contrast with the general colour of the stone,
and impart to it a weird effect (Plate XXIX, Fig. 15). Cut stones are often
confused with tourmalines, and can, indeed, only be distinguished from the
latter with certainty by noting on the refractometer the smaller amount of
double refraction and the difference in its character. The least and greatest
of the refractive indices are 1·62 and 1·643 respectively, and the double
refraction, 0·011, about half that of tourmaline, is biaxial and negative; the
specific gravity is 3·18, and hardness 7½ on Mohs’s scale.
Good stones are found at Minas Novas, Minas Geraes, Brazil, and in the
gem-gravels of Ceylon. It was first known from the province of Andalusia,
Spain, whence is the origin of its name.

Idocrase
(Vesuvianite, Californite)
Idocrase, also known as vesuvianite, is occasionally found in the form of
transparent, leaf-green, and yellowish-brown stones which, when cut, may
be mistaken for diopside and epidote respectively, but are distinguishable
from both by the extreme smallness of their double refraction. Californite is
a compact variety which has all the appearances of a jade; its colour is
green, or nearly colourless with green streaks.
In composition idocrase is a silicate of aluminium and calcium, the
precise formula of which is uncertain, but may be—

(Ca,Mn,Mg,Fe)2[(Al,Fe)(OH,F)]Si2O7.
The double refraction, which is uniaxial in character and negative in sign,
may be less than 0·001, and never exceeds 0·006, so that it is not easily
detected with the refractometer, even in sodium light. The refractive indices
vary enormously in value, from 1·702 to 1·726 for the ordinary, and from
1·706 to 1·732 for the extraordinary ray. The specific gravity varies from
3·35 to 3·45, and the hardness is about 6½ on Mohs’s scale.
The name idocrase, from εἴδος, form, and κρᾶσις, mixture, was assigned
to the species by Haüy, but his reasons have little meaning at the present
day. The other names are taken from the localities where the species and the
variety were first discovered.
Bright, green crystals come from Russia, and also from Ala Valley,
Piedmont, and Mount Vesuvius, Italy. Californite is found in large masses in
Siskiyon and Fresno Counties, California.

Epidote
(Pistacite)
Epidote often possesses a peculiar shade of yellowish green, similar to
that of the pistachio-nut—hence the origin of its alternative name—which is
unique among minerals, though scarcely pleasing enough to recommend it
to general taste. Its ready cleavage renders it liable to flaws; nevertheless, it
is occasionally faceted. The name epidote, from ἐπίδοσις, increase, was
given to it by Haüy, but not on very precise crystallographical grounds.
In composition this species is a silicate of calcium and aluminium, with
some ferric oxide in place of alumina, corresponding to the complex
formula, Ca2(Al,Fe)2[(Al,Fe)OH](SiO4)3. It occurs in monoclinic, prismatic
crystals richly endowed with natural faces. The colour deepens with
increase in the percentage amount of iron, and the stones become almost
opaque. The double refraction is large in amount, 0·031, biaxial in
character, and negative in sign. The dichroism is conspicuous in transparent
stones, the twin-tints corresponding to the principal optical directions being
green, brown, and yellow. The values of the least and greatest of the
refractive indices given by transparent stones are 1·735 and 1·766

respectively; the specific gravity varies from 3·25 to 3·50, and the hardness
from 6 to 7 on Mohs’s scale.
Transparent crystals have come from Knappenwand, Untersulzbachtal,
Salzburg, Austria; Traversella, Piedmont, Italy; and Arendal, Nedenäs,
Norway. Magnificent, but very dark, crystals were discovered about ten
years ago on Prince of Wales Island, Alaska.

Sphene
(Titanite)
The clear, green, yellow, or brownish stones provided by this species
would be welcomed in jewellery because of their brilliant and almost
adamantine lustre, but, unfortunately, they are too soft to withstand much
wear, the hardness being only 5½ on Mohs’s scale. In composition sphene
is a silico-titanate of calcium corresponding to the formula CaTiSiO5, and
in this respect comes near the recently discovered gem-stone, benitoite. The
refractive indices lie outside the range of the refractometer, the values of the
least and the greatest of the refractive indices varying from 1·888 and 1·917
to 1·914 and 2·053 respectively. It is to this high refraction that it owes its
brilliant lustre. The double refraction, which is biaxial in character and
positive in sign, is so large that the apparent doubling of the opposite edges
of a cut stone when viewed through one of the faces is obvious to the
unaided eye (cf. p. 41). Cut stones have additional interest on account of the
vivid dichroism displayed, the twin-tints, colourless, yellow, and reddish
yellow, corresponding to the three principal optical directions, being in
strong contrast. The specific gravity ranges from 3·35 to 3·45. The negative
test with the refractometer (cf p. 26), the softness, and the large amount of
double refraction suffice to distinguish this species from gem-stones of
similar appearance.
The name sphene, from σφήν, wedge, alludes to the shape of the natural
crystals. The alternative name is obviously due to the fact that the species
contains titanium.
Good stones have come from the St. Gothard district, Switzerland.

Axinite

Called axinite from the shape of its crystals—ἀξίνη, axe—this species
supplies small, clear, clove-brown, honey-yellow, and violet stones which
can be cut for those who care for a stone out of the ordinary. The
composition is a boro-silicate of aluminium and calcium, with varying
amounts of iron and manganese, corresponding to the formula
(Ca,Fe)3Al2(B.OH)Si4O15. Axinite is interesting on account of its strong
dichroism, the twin-tints corresponding to the principal optical directions
being violet, brown, and green. The double refraction is biaxial in character
and negative in sign, the least and greatest of the refractive indices being
1·674 and 1·684; the specific gravity is 3·28, and hardness about 6½ to 7,
or rather under that of quartz.
The best examples have been found at St. Cristophe, Bourg d’Oisans, in
the Dauphiné, France. Violet axinite is a novelty that has come within
recent years from Rosebery, Montagu County, Tasmania.

Prehnite
This species, which is named after its discoverer, Colonel Prehn, is found
in nodular, yellow and oil-green stones, of which the latter have very
occasionally been cut. It is a little soft, the hardness being only 6 on Mohs’s
scale. The double refraction is large in amount, 0·033, biaxial in character,
and positive in sign, the least and the greatest of the refractive indices being
1·616 and 1·649 respectively; the specific gravity varies from 2·81 to 2·95.
In composition prehnite is a silicate of aluminium and calcium
corresponding to the formula H2Ca2Al2(SiO4)3.
The best material has been found at St. Cristophe, Bourg d’Oisans,
Dauphiné, France.

Apatite
This interesting mineral is found occasionally in attractive green, blue, or
violet stones, but is unfortunately too soft for extensive use in jewellery, the
hardness being only 5 on Mohs’s scale. In composition it is a fluo-chloro-
phosphate of calcium, corresponding to the formula Ca4[Ca(F,Cl)](PO4)3.
When pure, it is devoid of colour, the tints being due to the presence of
small amounts of tinctorial agents. The double refraction is uniaxial in
character and negative in sign, the ordinary index being 1·642 and the

extraordinary 1·646; the specific gravity varies from 3·17 to 3·23. The
dichroism is usually feeble, but sometimes is strong; for instance, in the
stones from the Burma ruby mines (yellow, blue-green). A cut stone might
be mistaken for tourmaline, but is distinguished by its softness, or, when
tested on the refractometer, by its inferior double refraction. It received its
name from ἀπατάειν, deceive, because it was wrongly assigned to at least
half a dozen different species in early days. Moroxite is a name sometimes
given to blue-green apatite.
Beautiful violet stones are found at Ehrenfriedersdorf, Saxony;
Schlaggenwald, Bohemia; and Mount Apatite, Auburn, Androscoggin
County, Maine, U.S.A.; and blue stones come from Ceylon.

Dioptase
Though of a pretty, emerald-green colour, dioptase has never been found
in large enough crystals for gem purposes, and it is, moreover, rather soft,
the hardness being only 5 on Mohs’s scale, and has an easy cleavage. In
composition it is a hydrous silicate of copper corresponding to the formula
CuH2SiO4. The double refraction, which is large in amount, is uniaxial in
character, and positive in sign, the ordinary refractive index being 1·667
and the extraordinary 1·723. Its colour and softness distinguish it from
peridot or diopside, which have about the same refractivity. The name was
assigned to the species by Haüy, from διὰ, through, and ὄπτομαι, see,
because the cleavage directions were distinguishable by looking through the
stone.
Dioptase has been found near Altyn-Tübe in the Kirghese Steppes, at
Rezbánya in Hungary, and Copiapo in Chili, and at the mine Mindouli, near
Comba, in the French Congo.

CHAP T E R XXXVI I

CASSITERITE, ANATASE, PYRITES, HEMATITE

Cassiterite

T HOUGH usually opaque, this oxide of tin, corresponding to the formula
SnO2, has occasionally, but very rarely, been found in small,
transparent, yellow and reddish stones suitable for cutting. The lustre is
adamantine. The refraction is uniaxial in character and positive in sign, the
ordinary index being 1·997 and extraordinary 2·093. The specific gravity is
high, ranging from 6·8 to 7·1. The hardness is on the whole less than that of
quartz, being about 6 to 7 on Mohs’s scale.

Anatase
This mineral, which is one of the three crystallized forms of titanium
oxide, TiO2, occurs often in small, brown, transparent stones which
occasionally find their way into the market. The lustre is adamantine. The
refraction is uniaxial in character and negative in sign, the extraordinary
index being 2·493 and ordinary 2·554. The specific gravity varies from
3·82 to 3·95, and the hardness is about 5½ to 6 on Mohs’s scale.

Pyrites, Hematite
These two metallic minerals were employed in ancient jewellery. The
former, sulphide of iron, FeS2, is brass-yellow in colour, and has a specific
gravity 5·2, and hardness 6½ on Mohs’s scale. It is found, when fresh, in
brilliant cubes. The latter, oxide of iron, Fe2O3, has a black metallic lustre,
but, when powdered, is red in colour—a mode of distinguishing it from
other minerals of similar appearance. Its specific gravity is 5·3, and
hardness 6½ on Mohs’s scale. In modern times it has been cut in spherical
form to imitate black pearls, but can easily be recognized by its greater
density and hardness. Hematite is used for signet stones, often with an
intaglio engraving.

CHAP T E R XXXVI I I

OBSIDIAN, MOLDAVITE

T WO forms of natural glass have been employed for ornamental
purposes. Obsidian results from the solidification without
crystallization of lava, and corresponds in composition to a granite. The
structure is seldom clear and transparent, and usually contains inclusions or
streaks. The colour is in the mass jet-black, but smoky in thin fragments,
and occasionally greenish. Its property of breaking with a keen cutting
edge, in the same way as ordinary glass, rendered it of extreme utility to
primitive man, who was ignorant of the artificial substance. The refraction
is, of course, single, and the refractive index approximates to 1·50. The
specific gravity varies from 2·3 to 2·5. The hardness is 5 on Mohs’s scale,
the same as ordinary glass.
Obsidian is obtained wherever there has been volcanic activity. Vast
mines of great antiquity exist in the State of Hidalgo, Mexico.
Moldavite, which differs in no respect from ordinary green bottle-glass,
is of interest on account of its problematical origin. Its occurrence in
various parts of Bohemia and Moravia cannot be explained as the result of
volcanic agency. It may possibly be the product of old and forgotten glass
factories which at one time existed on the site. Even meteorites have been
suggested as the source. The physical characters are the same as those of
ordinary glass: refraction single, index 1·51; specific gravity 2·50 and
hardness 5½ on Mohs’s scale. Moldavite also passes under the names of
bottle-stone, or water-chrysolite. A natural glass of the same character has
been found in water-worn fragments in Ceylon, and has been sold as
peridot, which it resembles in colour, but is readily distinguished from it by
its very different physical properties.

PART II—SECTION C

ORNAMENTAL STONES

CHAP T E R XXXI X

FLUOR, LAPIS LAZULI, SODALITE, VIOLANE,
RHODONITE, AZURITE, MALACHITE, THULITE,
MARBLE, APOPHYLLITE, CHRYSOCOLLA, STEATITE
OR SOAPSTONE, MEERSCHAUM, SERPENTINE

S PACE will not permit of more than a few words concerning the more
prominent of the numerous mineral species which are employed for
ornamental purposes in articles of virtu or in architecture, but which for
various reasons cannot take rank as gem-stones.
Fluor, a beautiful mineral which is found in its greatest perfection in
England, has enjoyed well-deserved popularity when worked into vases or
other articles. The finest material, deep purple in colour, known as ‘Blue
John,’ came from Derbyshire, but the supply is now exhausted. The
crystallized examples, from Durham, Devonshire, and Cornwall, form some
of the most attractive of museum specimens. The crystals take the shape of
cubes, often twinned, and have an easy octahedral cleavage. The refraction
is single, the index being 1·433. Fluor is noted for its property of appearing
of differing colour by reflected and transmitted light, and the phenomenon
is in consequence known as fluorescence. The specific gravity is 3·18, and
the hardness 4 on Mohs’s scale. Owing to its low refraction and softness,
fluor is not suitable for jewellery. Clear colourless material is in demand for
particular lenses of microscope objectives.
The lovely blue stone known as lapis lazuli has since the earliest times
been applied to all kinds of decorative purposes, for mosaic and inlaid work
and as the material for vases, boxes, and so on, and was the original
sapphire of the ancients. When ground to powder it furnishes a fine blue
paint, but it has now been entirely superseded for this purpose by an
artificial product. Although to the eye so homogeneous and uniform in
structure, lapis lazuli has been shown by microscopic examination to be
composed of calcite coloured by three blue minerals in varying proportions.
All three belong to the cubic class of symmetry, and are mainly soda

aluminium silicates in composition; their hardness varies from 5 to 6 on
Mohs’s scale. Lazurite, Na4(NaS3.Al)Al2Si3O12, has specific gravity
varying from 2·38 to 2·45, and hardness about 5 to 5½; haüynite,
(Na2,Ca)2(NaSO4,Al)Al2Si3O12, is about the same in specific gravity, 2·4 to
2·5, but slightly harder, 5½ to 6; while sodalite, Na4(AlCl)Al2Si3O12, is the
lightest in density, 2·14 to 2·30, with hardness 5½ to 6, and has a refractive
index 1·483.
By far the oldest mines are in the Badakshan district of Afghanistan, a
few miles above Firgamu in the valley of the Kokcha, a branch of the Oxus,
where ruby and spinel are found. It is also found at the southern end of
Lake Baikal, Siberia, and in the Chilian Andes.
Sodalite occurs in beautiful blue masses at Dungannon, Hastings County,
Ontario, Canada, and at Litchfield, Maine, U.S.A. They make excellent
polished stones.
Violane, a massive, dark violet-blue diopside from San Marcel,
Piedmont, Italy, also makes a handsome polished stone.
Rhodonite, silicate of manganese, MnSiO3, possesses a fine red colour,
and makes an attractive stone when cut and polished. It has very slight
biaxial double refraction, the refractivity being about 1·73; the specific
gravity is 3·6, and hardness 6. It is found in large masses near Ekaterinburg
in the Ural Mountains, and is quarried as an ornamental stone.
Both the copper carbonates, azurite or chessylite, and malachite, make
effective polished stones. The latter is also worked into various ornamental
objects; it occurs in fibrous masses, the grained character of which look
well in the polished section. Its colour is a bright green, to which it owes its
name, from μαλακή, mallows. Its composition is represented by the formula
CuCO3.Cu(OH)2, and it is the more stable form, since azurite is frequently
found altered to it. It has biaxial double refraction, and the indices are about
1·88; the specific gravity is 4·01, and hardness about 3½ to 4 on Mohs’s
scale. It is found in large masses at the copper mines of Nizhni Tagilsk in
the Ural Mountains, where it is mined as an ornamental stone; it also
accompanies the copper ores in many parts of the world, for instance Cuba,
Chili, and Australia. Azurite, so called on account of its beautiful blue
colour, is rarer, but, unlike malachite, is generally in the form of crystals.

Beautiful specimens have come from Chessy, near Lyons, France, and
Bisbee, Arizona, U.S.A. The composition corresponds to the formula
2CuCO3, Cu(OH)2. The specific gravity is 3·80, and hardness about 3½ to
4.
Chrysocolla occurs in blue and bluish-green earthy masses, with an
enamel-like texture, which in some instances can be worked and polished.
Being the result of the decomposition of copper ores, it varies considerably
in hardness, ranging from 2 to 4 on Mohs’s scale. Its composition
approaches to the formula CuSiO3.2H2O, but it invariably contains
impurities. It is very light, the density being only about 2·2.
Steatite, or soapstone, is a massive foliated silicate of magnesium
corresponding to the formula H2Mg3Si4O12, which is one of the softest of
mineral substances, representing the degree 1 on Mohs’s scale, but in
massive pieces is harder owing to the intermixture of other substances with
it. It has a peculiar greasy feeling to the touch, due to its softness. The
specific gravity is about 2·75. The Chinese carve images out of the
yellowish and brownish pieces.
Meerschaum, a silicate of magnesium corresponding to the formula
H4Mg2Si3O10, is familiar to every smoker as a material for pipe-bowls. It is
very light, the specific gravity being only 2·0, and soft, the hardness being
about 2 to 2½ on Mohs’s scale. When found, it is pure white in colour, and
answers to its name, a German word signifying sea-foam. It comes from
Asia Minor.
Serpentine has been largely used for decorative purposes, as well as for
cameos and intaglios, and formed most of the famous ‘verde antique.’
Being the result of the decomposition of other silicates it varies enormously
in appearance and characters, but the most attractive stones are a rich oil-
green in colour and resemble jade. The composition approximates to the
formula H4Mg3Si2O9, but it invariably contains other elements. The
hardness varies from 2½ to 4 on Mohs’s scale, according to the minerals
contained in the stone; the specific gravity is about 2·60 and the refractivity
1·570.
The beautiful rose-red stone, thulite, makes a handsome decorative stone.
It has nearly the same composition as epidote (p. 275), and like it has strong

dichroism, the principal colours being yellow, light rose, and deep rose. The
colour is due to manganese. Its refractive index is about 1·70, specific
gravity 3·12, and hardness 6 to 6½ on Mohs’s scale; it possesses an easy
cleavage. Fine specimens come from Telemark, Norway, and it is therefore
called after the old name for Norway, Thule.
Marble is a massive calcite, carbonate of lime, with the formula CaCO3.
When pure it is white, but it is usually streaked with other substances which
impart a pleasing variety to its appearance. It is always readily recognized
by the immediate effervescence set up when touched with a drop of acid.
Calcite is highly doubly refractive (cf. p. 40), the extraordinary index being
1·486, and ordinary 1·658, a difference of 0·172; the specific gravity is
2·71, and hardness 3 on Mohs’s scale. Lumachelle, or fire-marble, is a
limestone containing shells from which a brilliant, fire-like chatoyancy is
emitted when light is reflected at the proper angle. It sometimes resembles
opal-matrix, but is easily distinguished by its lower hardness and by its
effervescent action with acid. Choice specimens come from Bleiberg in
Carinthia, and from Astrakhan.
Apophyllite has not many characters to commend it, being at the best
faintly pinkish in colour, and always imperfectly transparent. It is a hydrous
silicate of potassium and calcium with the complex formula
(H,K)2Ca(SiO3)2.H2O. Its refractivity is about 1·535, specific gravity 2·5,
and hardness 4½ on Mohs’s scale; it possesses an easy cleavage. It occurs in
the form of tetragonal crystals at Andreasberg in the Harz Mountains, and in
the Syhadree Mountains, Bombay, India.

PART II—SECTION D

ORGANIC PRODUCTS

CHAP T E R XL

PEARL, CORAL, AMBER

A LTHOUGH none of the substances considered in this chapter come
within the strict definition of a stone, since they are directly the result
of living agency, yet pearl at least cannot be denied the title of a gem. Both
pearl and coral contain calcium carbonate in one or other of its crystallized
forms, and both are gathered from the sea; but otherwise they have nothing
in common. Amber is of vegetable origin, and is a very different substance.

Pearl
From that unrecorded day when some scantily clothed savage seeking for
succulent food opened an oyster and found to his astonishment within its
shell a delicate silvery pellet that shimmered in the light of a tropical sun,
down to the present day, without intermission, pearl has held a place all its
own in the rank of jewels. Though it be lacking in durability, its beauty
cannot be disputed, and large examples, perfect in form and lustre, are
sufficiently rare to tax the deepest purse.
The substance composing the pearl is identical with the iridescent lining
—mother-o’-pearl or nacre, as it is termed—of the shell. Tortured by the
intrusion of some living thing, a boring parasite, a worm, or a small fish, or
of a grain of sand or other inorganic substance, and without means to free
itself, the mollusc perforce neutralizes the irritant matter by converting it
into an object of beauty that eventually finds its way into some jewellery
cabinet. Built up in a haphazard manner and not confined by the inexorable
laws of intermolecular action, a pearl may assume any and every variety of
shape from the regular to the fantastic. It may be truly spherical, egg- or
pear-shaped—pear-drops or pear-eyes, as they are termed—or it may be
quite irregular—the so-called baroque or barrok pearls. The first is the most
prized, but a well-shaped drop-pearl is in great demand for pendants or ear-
rings. The colour is ordinarily white, or faintly tinged yellowish or bluish,
and somewhat rarely, salmon-pink, reddish, or blackish grey. Perfect black
pearls are valuable, but not as costly as the finest of the white. Though not

transparent, pearl is to a varying extent translucent, and its characteristic
lustre—‘orient’ in the language of jewellery—is due to the same kind of
interaction of light reflected from different layers that has been remarked
upon in the case of opal and certain other stones. The translucency varies in
degree, and some jewellers speak of the ‘water’ of pearls just as in the case
of diamonds. If a pearl be sliced across the middle and the section be
examined under the microscope, it will be seen that the structure consists of
concentric shells and resembles that of an onion. These shells are alternately
composed of calcium carbonate in its crystallized form, aragonite, and of a
horny organic matter known as conchiolin, and they evidently represent the
result of intermittent growth. Because of their composite character, pearls
have a specific gravity ranging from 2·65 to 2·69–2·84–2·89 in the case of
pink pearls—which is appreciably less than that of aragonite, 2·94: the
hardness is about the same, namely, 3½ to 4 on Mohs’s scale. That the
arrangement of the mineral layers is approximately parallel is evinced by
the distinctness of the shadow-edges shown on examination with the
refractometer. Pearls require very careful handling, both because they are
comparatively soft and therefore apt to be scratched, and because they are
chemically affected by acids, and even by the perspiration from the skin.
Acids attack only the calcium carbonate, not the organic matter; the well-
known story therefore of Cleopatra dissolving a valuable pearl in vinegar,
which is moreover, too weak an acid to effect the solution quickly, must not
be accepted too literally. Pearls are not cut like stones, and therefore as soon
as the precious bloom has once gone, nothing can be done to revive it.
Attempts are sometimes made in the case of valuable pearls to remove the
dull skin and lay bare another iridescent layer underneath, but the operation
is exceedingly delicate. Even with the best of care pearls must in process of
time perish owing to the decay of the organic constituent. Pearls that have
been discovered in ancient tombs crumbled to dust at a touch, and those
formerly in ancient rings have vanished or only remain as a brown powder,
while the garnets or other stones set with them are little the worse for the
centuries that have passed by.
The largest known pearl was at one time in the famous collection
belonging to the banker, Henry Philip Hope. Cylindrical in form, with a
slight swelling at one end, it measures 50 mm. (2 inches) in length, and 115
mm. (4½ inches) in circumference about the thicker, and 83 mm. (3¼
inches) about the thinner end, and weighs 454 carats. About three-quarters

of it is white in colour with a fine ‘orient,’ and the remainder is bronze in
tint. It is valued at upwards of £12,000. A large pearl, 300 carats in weight,
is in the imperial crown of the Emperor of Austria, and another, pear-
shaped, is in the possession of the Shah of Persia. A beautiful white India
pearl, a perfect sphere in shape, and 28 carats in weight, is in the Museum
of Zosima in Moscow; it is known as ‘La Pellegrina.’ The ‘Great Southern
Cross,’ which consists of nine large pearls naturally joined together in the
shape of a cross, was discovered in an oyster fished up in 1886 off the beds
of Western Australia. The collection of jewels in the famous Green Vaults at
Dresden contains a number of pearls of curious shapes.
Large pearls are sold separately, while the small pearls known as ‘seed’
pearls come into the market bored and strung on silk in ‘bunches.’ The unit
of weight is the pearl grain, which is a quarter of a carat, and the rate of
price depends on the square of the weight in grains. The rate per unit or
base varies from 6d. to 50s. according to the shape and quality of the pearl.
Spherical pearls command the best prices, next the pearl-drops, and lastly
the buttons; but whatever the shape, it is imperative that the pearl have
‘orient,’ without which it is valueless. The cheaper grades of pearls are sold
by the carat.

PLATE XXX

NATIVES DRILLING PEARLS

For use in necklaces and pendants pearls are bored with a steel drill, and
threaded with silk, an easy operation on account of their softness. They
harmonize well with diamonds. Small pearls are often set as a frame to
large coloured stones, to which they form an admirable foil. Pearls set in
rings or anywhere where the upper half alone would show are generally
sawn in halves; ‘button’ pearls find an extensive use in modern rings.
Any mollusc, whether of the bi-valve or the uni-valve type, which
possesses a nacreous shell, has the power of producing pearls, but only two,
the pearl-oyster, Meleagrina margaritifera, and the pearl-mussel, Unio
margarifer, repay the cost of systematic fishing. The outside of the shell is
formed of the horny matter called conchiolin; while the inside is composed
of two coats, of which the outer consists of alternate layers of conchiolin
and calcium carbonate in its crystallized form, calcite, and the inner of the
same organic matter, but with calcium carbonate in its other crystallized
form, aragonite. The latter coat forms the nacreous lining known as mother-
o’-pearl, which is identical in consistency with pearl, but somewhat more
transparent. The iridescence of mother-o’-pearl is due not only to the fact
that it is composed of a succession of thin translucent layers, but also to the

fact that these layers overlap like slates on a house, and form a series of fine
parallel lines on the surface; diffraction therefore as well as interference of
light takes place, and a similar diffraction phenomenon is displayed even by
a cast of the inside of the shell. The animal has the property of secreting
calcium carbonate, which it absorbs from the sea-water, in both its
crystallized conditions as well as conchiolin. At the outer rim it secretes
conchiolin, further in calcite, and at the very inside aragonite. The shape
and appearance of a pearl therefore depend on the position in which the
intruding substance is situated within the shell. The most perfect pearl has
been in intermittent motion in the interior of the mollusc, and has received
successive coats according to the position in which it happened to be. A
parasite that bores into the shell is walled up at the point of entrance, and a
wart- or blister-pearl results. The thinner the successive coats the finer the
lustre. Pearls have even been discovered embedded in the animal itself. The
number of pearls found in a shell depends on the number of times the living
host was compelled to seal up some irritant object, and may vary from one
up to the eighty-seven which are said to have been found in an Indian
oyster. That an oyster thus distinguished has not led a happy existence is
testified by the distorted shape of its shell, a clue that guides the pearl-
fishers in their search. Moreover, pearl-oysters never have thick nacreous
shells, and on the other hand molluscs with fine mother-o’-pearl seldom
contain pearls.
Beautiful white and silvery pearls are found in a small oyster that lives at
a depth of 6 to 13 fathoms (11–24 m.) in the Gulf of Manaar, off the coast
of Ceylon. About seven-eighths, however, of the pearls that come into the
market are obtained from a larger oyster which has its home on the Arabian
coast of the Persian Gulf. These famous fisheries have been known since
very early times. The pearls found here are more yellowish than those from
Ceylon, but are nevertheless of excellent quality. The pearl fisheries off the
north-west coast of Western Australia and off Venezuela are also not
unimportant, and fine black pearls have been supplied by molluscs from the
Gulf of Mexico.

PLATE XXXI

METAL FIGURES OF BUDDHA INSERTED IN A PEARL-OYSTER

PLATE XXXII

FIG. 1

FIG. 2

SECTIONS OF CULTURE PEARL
FIG. 1. IN THE OYSTER. FIG. 2. WHEN FINISHED.
A. PEARLY DEPOSIT. B. PIECE OF MOTHER-O’-PEARL INSERTED IN THE
OYSTER.
C. OUTER SHELL OF THE OYSTER. D. MOTHER-O’-PEARL BACK ADDED.

The Chinese have long made a practice of introducing into the shell of a
pearl-oyster little tin images of Buddha in order that they may be coated
with the nacreous secretion. The Japanese have during recent years made
quite an industry of stimulating the efforts of the mollusc by cementing
small pieces of mother-o’-pearl to the interior surface of the shell (Plate
XXXII, Fig. 1); these ‘culture’ pearls, as they are termed, are recognizable
by examination of the back. About a year has to elapse before a coating of a
tenth of a millimetre is formed, and another two years must pass before the
thickness is doubled. After removal the piece of mother-o’-pearl, which is

now coated with several nacreous layers, is cemented to a piece of ordinary
mother-o’-pearl, and the lower portion is ground to the usual symmetrical
shape (Plate XXXII, Fig. 2). Blister pearls are often similarly treated. In
both cases, however, the ‘orient’ is deficient in quality.
The finest mother-o’-pearl is supplied by a mollusc found in the sea near
the islands lying between Borneo and the Philippines, and fine material is
found at Shark Bay and off Thursday Island.

Coral
Coral ranks far below pearl and meets with but limited appreciation. It is
common enough in warm seas, but the only kind which finds its way into
jewellery is the rose or red-coloured coral—the noble coral, Corallium
nobile or rubrum. It consists of the axial skeleton of the coral polyp, and is
built up of hollow tubes fitting one within the other. The composition is
mainly calcium carbonate with a little magnesium carbonate and a small
amount of organic matter. The former of the mineral substances is in the
form of calcite, and the crystals are arranged in fibrous form radiating at
right angles to the axis of the coral. The specific gravity varies from 2·6 to
2·7, being slightly under that of calcite, and the hardness is somewhat
greater, being about 3¾ on Mohs’s scale.
The best red coral is found in the Mediterranean Sea off Algiers and
Tunis in Africa, and Sicily and the Calabrian Coast of Italy. The industry of
shaping and fashioning the coral is carried on almost entirely in Italy. Coral
is usually cut into beads, either round or egg-shaped, and used for
necklaces, rosaries, and bracelets. The best quality fetches from 20s. to 30s.
per carat.

Amber
This fossil resin, yellow and brownish-yellow in tint, finds an extensive
use as the material for mouthpieces of pipes, cigar and cigarette-holders,
umbrella-handles, and so on, and is even locally cut for jewellery, although
its extreme softness, its hardness being only 2½ on Mohs’s scale, quite
unfits it for such a purpose. It is only slightly denser than water, the specific
gravity being about 1·10. Since the structure is amorphous the refraction is
single, the index being about 1·540. Amber, being a very bad conductor of

heat, is perceptibly warm to the touch. Its property of becoming electrified
by friction attracted early attention, and from the Greek name for it,
ἤλεκτρον, is derived our word electricity.
Amber is washed up by the sea off the coasts of Sicily and Prussia, and of
Norfolk and Suffolk in England. The finest examples, which are picked up
off the shore of Catania in Sicily, are distinguished by a fine bluish
fluorescence, resembling that seen in lubricating oil; such pieces command
good prices.
A recent resin, pale yellow in colour, known as kauri-gum, is found in
New Zealand, where it is highly valued.

TABLES

TABL E I

Chemical Composition of Gem-Stones
(a) Elements—
Diamond C
(b) Oxides—
Corundum Al2O3
Quartz SiO2
Chalcedony SiO2
Opal SiO2.nH2O
(c) Aluminates—
Spinel MgAl2O4
Chrysoberyl BeAl2O4
(d) Silicates—
Phenakite Be2SiO4
Dioptase H2CuSiO4
Peridot Mg2SiO4
Zircon ZrSiO4
Enstatite MgSiO3
Diopside CaMg(SiO3)2
Nephrite CaMg3(SiO3)4
Sphene CaTiSiO5
Benitoite BaTiSi3O9
Andalusite Al(AlO)SiO4

Kyanite (AlO)2SiO3
Topaz [Al(F,OH)]2SiO4
Epidote Ca2(Al,Fe)2(AlOH)(SiO4)3
Euclase Be(AlOH)SiO4
Prehnite H2Ca2Al2(SiO4)3
Iolite H2(Mg,Fe)4Al8Si10O37
Hessonite Ca3Al2(SiO4)3
Pyrope Mg3Al2(SiO4)3
Garnet { Almandine Fe3Al2(SiO4)3
Andradite Ca3Fe2(SiO4)3
Beryl Be3Al2(SiO3)6
Spodumene LiAl(SiO3)2
Jadeite NaAl(SiO3)2
Moonstone KAlSi3O8
12SiO2.3B2O3.(9-x)[(Al,Fe)2O3].3x[(Fe,
Tourmaline
Mn,Ca,Mg,K2,Na2,Li2,H2)O].3H2O
Axinite HCa3Al2B(SiO4)4
Idocrase (Ca,Mn,Mg,Fe)2(Al,Fe)(OH,F)]Si2O7
(e) Phosphates—
Beryllonite NaBePO4
Apatite Ca5(F,Cl)(PO4)3
Turquoise CuOH.6[Al(OH)2].H5.(PO4)4

TABL E I I

Colour of Gem-Stones
Colourless and White.—Diamond, corundum (white sapphire), topaz,
quartz (rock-crystal), zircon (when ‘fired’), moonstone; rarely beryl,
tourmaline; among the less common species, phenakite, spodumene
(colourless kunzite), beryllonite.
Yellow.—Diamond, topaz, corundum (yellow sapphire), quartz (citrine,
Scotch or occidental topaz), tourmaline, zircon, sphene, spodumene,
beryl.
Pink and Lilac.—Corundum (pink sapphire), spinel (balas-ruby),
tourmaline (rubellite), topaz (usually when ‘fired’), spodumene
(kunzite), beryl (morganite), quartz (rose-quartz).
Red.—Corundum (ruby), garnet (pyrope, almandine), spinel (balas-ruby),
tourmaline (rubellite), zircon, opal (fire-opal).
Green.—Beryl (emerald, aquamarine), peridot, corundum, tourmaline,
chrysoberyl (including alexandrite), zircon, garnet (demantoid); among
less common species, spodumene (hiddenite), euclase, diopside,
idocrase, epidote, apatite, obsidian; rarely diamond; also semi-opaque,
turquoise, jade.
Blue.—Corundum (sapphire), spinel, topaz, tourmaline, zircon; among the
less common species, kyanite, iolite, benitoite, apatite; rarely diamond;
also semi-opaque, turquoise, lapis lazuli, sodalite.
Violet and Purple.—Quartz (amethyst), corundum (oriental amethyst),
spinel (almandine-spinel), garnet (almandine), spodumene (kunzite),
apatite.
Brown.—Diamond, tourmaline, quartz (smoky-quartz); among the less
common species, andalusite, axinite, sphene.

TABL E I I I

Refractive Indices of Gem-Stones[8]
Opal 1·454
Moonstone 1·53 1·54
Iolite 1·543 1·551
Quartz 1·544 1·553
Beryllonite 1·553 1·565
Beryl 1·578 1·585
Turquoise 1·61 1·65
Topaz 1·618 1·627
Andalusite 1·632 1·643
Tourmaline 1·626 1·651
Apatite 1·642 1·646
Phenakite 1·652 1·667
Euclase 1·651 1·670
Spodumene 1·660 1·675
Enstatite 1·665 1·674
Peridot 1·659 1·697
Axinite 1·674 1·684
Diopside 1·685 1·705
Idocrase 1·714 1·719
Spinel 1·726
Kyanite 1·72 1·73
Epidote 1·735 1·766
Garnet (Hessonite) 1·745
Chrysoberyl 1·746 1·753
Garnet (Pyrope) 1·755
Benitoite 1·757 1·804
Corundum 1·761 1·770

Garnet (Almandine) 1·790
Zircon (a) 1·815
Garnet (Demantoid) 1·885
Sphene 1·901 1·985
Zircon (b) 1·927 1·980
Diamond 2·417

TABL E I V

Colour-Dispersion of Gem-Stones[9]
Moonstone ·012
Quartz ·013
Beryl ·014
Topaz ·014
Chrysoberyl ·015
Tourmaline ·017
Spodumene ·017
Corundum ·018
Peridot ·020
Spinel ·020
Garnet (Almandine) ·024
Garnet (Pyrope) ·027
Garnet (Hessonite) ·028
Zircon ·038
Diamond ·044
Sphene ·051
Garnet (Demantoid) ·057

TABL E V

Character of the Refraction of Gem-Stones
(a) Single—
Diamond, spinel, garnet, opal.
Diamond and garnet frequently display local double refraction.

(b) Uniaxial, Positive—
Quartz ·009
Phenakite ·015
Benitoite ·047
Zircon (b) ·053
Quartz exhibits circular polarization.

(c) Uniaxial, Negative—
Apatite ·004
Idocrase ·005
Beryl ·007
Corundum ·009
Tourmaline ·025

(d) Biaxial, Positive—
Chrysoberyl ·007
Topaz ·009
Enstatite ·009
Spodumene ·015
Euclase ·019
Diopside ·020
Peridot ·038
Sphene ·084

(e) Biaxial, Negative—
Moonstone ·006
Iolite ·008
Axinite ·010
Andalusite ·011
Beryllonite ·012
Kyanite ·016
Epidote ·031

TABL E VI

Dichroism of Gem-Stones
(a) Strong
Corundum, tourmaline, alexandrite, spodumene, andalusite, iolite, epidote,
axinite.
(b) Distinct
Emerald, topaz, quartz, peridot, chrysoberyl, enstatite, euclase, idocrase,
kyanite, sphene, apatite.
(c) Weak
Beryl, diopside.

TABL E VI I

Specific Gravities of Gem-Stones
Opal 2·15
Moonstone 2·57
Iolite 2·63
Quartz 2·66
Beryl 2·74
Turquoise 2·82
Beryllonite 2·84
Phenakite 2·99
Euclase 3·07
Tourmaline 3·10
Enstatite 3·10
Andalusite 3·18
Spodumene 3·18
Apatite 3·20
Axinite 3·28
Diopside 3·29
Epidote 3·37
Peridot 3·40
Idocrase 3·40
Sphene 3·40
Diamond 3·52
Topaz 3·53
Spinel 3·60
Kyanite 3·61
Garnet (Hessonite) 3·61
Benitoite 3·64
Chrysoberyl 3·73

Garnet (Pyrope) 3·78
Garnet (Demantoid) 3·84
Corundum 4·03
Garnet (Almandine) 4·05
Zircon (a) 4·20
Zircon (b) 4·69

TABL E VI I I

Degrees of Hardness of Gem-Stones
5. Kyanite (5–7), apatite, lapis lazuli
5½. Enstatite, beryllonite, sphene
6. Opal, moonstone, turquoise, diopside
Spodumene, peridot, garnet (demantoid), benitoite, idocrase,
6½.
epidote, axinite, jade (nephrite)
7. Iolite, quartz, tourmaline, jade (jadeite)
7¼. Garnet (hessonite, pyrope)
7½. Beryl, garnet (almandine), zircon, phenakite, euclase, andalusite
8. Topaz, spinel
8½. Chrysoberyl
9. Corundum
10. Diamond

TABL E I X .—Data

Densities of Water and Toluol at Ordinary Temperatures
Temperature Water Toluol
Centigrade Fahrenheit
14° 57·2° 0·9994 0·8697
15° 59·0° 0·9992 0·8687
16° 60·8° 0·9990 0·8677
17° 62·6° 0·9988 0·8667
18° 64·4° 0·9986 0·8657
19° 66·2° 0·9985 0·8647
20° 68·0° 0·9983 0·8637
21° 69·0° 0·9981 0·8627
22° 71·6° 0·9979 0·8617
23° 73·4° 0·9977 0·8607
1 English carat = 0·2053 gram
1 Metric carat = 0·2000 (one-fifth) gram
1 oz. Av. = 28·35 grams
1 lb. Av. = 0·4536 kilogram
1 inch = 25·4 millimetres
1 foot = 0·3048 metre
1 yard = 0·9144 metre
1 mile = 1·6093 kilometre

INDEX

Absorption, 53, 59
Absorption spectra, 59
Achroite, 220, 221
Adularia, 255
Agate, 247
Akbar Shah diamond, 163
Alalite, 272
Albite, 254
Alexandrite, 54, 60, 233
Scientific, 122
Almandine, 60, 214
Oriental, 112, 172
spinel, 112, 204
Amazon-stone, 255
Amber, 83, 298
Amethyst, 239, 242
Oriental, 111, 172, 239
Anatase, 281
Andalusite, 274
Andradite, 216
Anomalous refraction, 47
Anorthite, 254
Apatite, 279
Apophyllite, 290
Aquamarine, 184, 193
Arizona-ruby, 213
Artificial stones, 124
Asteria, 38, 177
Asterism, 38
Australia stones, 154, 174, 182, 195, 213, 216, 227, 232, 252, 288
Austrian Yellow diamond, 165
Aventurine, 240, 241
Axes, Crystallographic, 9
Optic, 49
Axinite, 278
Azure-quartz, 244
Azurite, 287

Balas-ruby, 203
Barnato, Barnett, 145
Baroque, Barrok, pearls, 292
Bastite, 272
Benitoite, 267
Berquem, Louis de, 90, 161
Beryl, 184
Beryllonite, 270
Bezel facet, 92
Biaxial double refraction, 45, 49, 57
Bisectrix, 45, 49
Black diamond, 129
Black lead, 129
Black opal, 249, 250
Black Prince’s ruby, 206
Blister-pearl, 296
Bloodstone, 247
Blue felspar, 255
Blue ground, 143, 147
Blue John, 285
Boart, 103, 129, 133
Bohemian garnet (pyrope), 207, 212
Bone turquoise, 259
Boodt, A. B. de, 132, 213
Borgis, Hortensio, 161
Borneo stones, 154, 170
Bort, v. Boart, 103, 129, 133
Bottle-stone, 284
Boule, 118
Bowenite, 263
Braganza diamond, 170
Brazil stones, 138, 165, 166, 169, 194 et seq., 201, 215, 223, 236, 243,
244, 248, 266, 269, 270, 274
Brazilian emerald, 111, 220, 221
peridot, 221
sapphire, 111, 221
topaz, 111, 197

Brilliant form of cutting, 92
Brilliant, Scientific, 122
Bristol diamonds, 243
Bruting, 100
Burma stones, 178, 205, 223, 227, 263
Button-pearl, 295
Byes, Bywaters, 136, 150

Cabochon form of cutting, 88
Cacholong, 251
Cairngorm, 239
Callaica, callaina, callais, 258
Calcite, 40, 289
California stones, 156, 195, 202, 224, 259, 265, 267, 275
Californite, 264, 275
Cape-ruby, 213
Carat weight, 72, 84
Carbon, 129
Carbonado, 129
Carborundum, 105
Carbuncle, 89, 215
Carnelian, 247
Cascalho, 139
Cassiterite, 281
Cat’s-eye (chrysoberyl), 38, 90, 233
(quartz), 39, 90, 240
(tourmaline), 39, 219
Hungarian, 244
Ceylon stones, 181, 195, 201, 205, 212, 215, 216, 223, 232, 236, 237,
243, 244, 255, 267, 274, 279, 284
Ceylonese peridot (tourmaline), 221
Ceylonite, 204
Chalcedony, 246
Chatoyancy, 38
Chert, 247
Chessylite, 287
Chrysoberyl, 233
Chrysocolla, 288

Chrysocolla, 288
Chrysolite (chrysoberyl), 233
(peridot), 225
Chrysoprase, 247
Church, Sir Arthur, 61, 211, 231
Cinnamon-stone, 211
Citrine, 239
Cleavage, 80, 100, 149
Close goods, 149
Colenso diamond, 131
Colour, 53
Colour dispersion, 20, 97
Conchiolin, 293
Coral, 298
Cordierite, 266
Cornish diamonds, 243
Corundum, 172
Crocidolite, 39, 240
Crookes, Sir William, 132, 153
Cross facet, 93
Crystal, 6, 7, 8
Rock-, 97
Cubic system, 8
Culet facet, 93
Cullinan diamond, 94, 100, 168
Culture pearls, 297
Cumberland diamond, 164
Cyanite (Kyanite), 79, 273
Cymophane, 234

Darya-i-nor diamond, 162
De Beers diamonds, 167
Demantoid, 216
Density, 63
Deviation, Minimum, 30
Diamond, Characters of, 128
cutting, 90
gauges, 86
Gl i ’ 135

Glaziers’, 135
mining, 146
Occurrence of, in—
Borneo, 154
Brazil, 139
German South-West Africa, 155
India, 138
New South Wales, 154
Rhodesia, 155
South Africa, 139
Origin of, 151
-point, 91
-rose, 92
-table, 91
Diamonds, Classification of, 136, 149
Historical, 157
Prices of, 135
Dichroism, 55
Dichroite, 266
Dichroscope, 55
Diffusion column, 65
Diopside, 272
Dioptase, 280
Dispersion, Colour, 20, 24, 97
Disthene, 273
Dop, 102
Double refraction, 28, 40
Doublet, 125
Dresden diamond, 171
Drop-stone, 94
Duke of Devonshire’s emerald, 191

Edwardes ruby, 175
Electrical characters, 82
Emerald, 89, 184
Brazilian, 220, 221
Evening, 225
Oriental, 111, 172

, ,
Scientific, 122
Uralian, 216
Emeraldine, 247
Emery, 175
English Dresden diamond, 166
Enstatite, 271
Epidote, 275
Essence d’Orient, 126
Essonite (Hessonite), 211
Euclase, 269
Eugénie diamond, 164
Evening emerald, 225
Excelsior diamond, 167
Extinction, 45

Faceting machine, 105
False topaz, 239
Felspar, 254
Fire, 20, 96
Fire-marble, 289
Fire-opal, 251
Flats, 150
Flêches d’amour, 240
Flint, 247
Floors, 147
Fluor, 285
Frémy, E., 115

Garnet, 207
Green, 271
Gaudin, M. A. A., 115
Gauges, Diamond, 86
Girdle, 92
Glass, 7, 124
Gnaga Boh ruby, 180
Goniometer, 30
Grain, Pearl, 86
Graphite 129