CHAPTER I

[Illustration:

_PLATE I_

_Frontispiece_

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

In 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

It 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_, _Jacinth_) 211

(_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_,

_Alexandrite_) 233

XXVIII. QUARTZ (_Rock-Crystal_, _Amethyst_, _Citrine_,

_Cairngorm_, _Cat’s-Eye_, _Tiger’s-Eye_) 238

XXIX. CHALCEDONY, AGATE, ETC. 246

XXX. OPAL (_White Opal_, _Black Opal_, _Fire-Opal_) 249

XXXI. FELSPAR (_Moonstone_, _Sunstone_, _Labradorite_,

_Amazon-Stone_) 254

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,

PREHNITE, APATITE, DIOPTASE 271

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,

CHRYSOCOLLA, STEATITE OR SOAPSTONE, MEERSCHAUM,

SERPENTINE 285

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 SAPPHIRES 118

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 PLOUGHING IT

OVER 146

XXII. WASHING-MACHINES FOR CONCENTRATING THE BLUE GROUND 147

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, BRAZIL 196

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-OYSTER 296

XXXII. SECTIONS OF CULTURE PEARL 297

GEM-STONES

CHAPTER I

INTRODUCTION

Beauty, 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