A Hand-Book of Precious Stones

Part 1

Transcriber’s Notes:

Underscores “” before and after a word or phrase indicate italics_ in the original text. Small capitals have been converted to SOLID capitals. Illustrations have been moved so they do not break up paragraphs. Old or antiquated spellings have been preserved. Typographical errors have been silently corrected but other variations in spelling and punctuation remain unaltered. Added “Spodumene 96” to TOC, as it was missing.

A HAND-BOOK OF PRECIOUS STONES

BY M. D. ROTHSCHILD

NEW YORK & LONDON G. P. PUTNAM’S SONS The Knickerbocker Press 1890

COPYRIGHT BY M. D. ROTHSCHILD 1889

The Knickerbocker Press, New York Electrotyped and Printed by G. P. Putnam’s Sons

CONTENTS.

PAGE What are Precious Stones? 7

Physical Characters— Crystallization 10 Cleavage 10 Fracture 11

Optical Properties— Refraction 12 Polarization of Light 13 Pleiochroism 14

Colors 15 Lustre 17 Streak 18 Hardness 19 Specific Gravity 21 Weight 27 Fusibility 28 Magnetism 30 Transparency 30 Phosphorescence 31 Electricity 31 Cutting and Polishing 32

Diamond 35 Corundum 39 The Ruby 40 Sapphire 43 Fancy Sapphires 44 Star Sapphires 45 Spinel 46 Beryl 50 Emerald 51 Beryl 53 Chrysoberyl 54 Cymophane 56 Alexandrite 56 Zircon 58 Turquois 60 Tourmaline 64 Opal 69 Pearl 71 Chrysolite 78 Garnet 80 Topaz 84 Apatite 87 Felspar 88 Moonstone 89 Sunstone (Avanturine Felspar) 90 Amazon Stone (Green Felspar) 91 Labradorite 91 Cyanite 93 Lapis Lazuli 94 Hiddenite 95 Spodumene 96 Dichroite 97 Idocrase 98 Euclase 99 Sphene 100 Phenacite 101 Epidote 101 Axinite 102 Diopside 103 Fluor Spar 104 Hypersthene 105 Quartz 106 Crystallized Quartz 109 Amethyst 110 Yellow Quartz 111 Cairngorm, etc 111 Rose Quartz 113 Avanturine 114 Cat’s-Eye 114 Crocidolite 115 Heliotrope 116 Chrysoprase 117 Prase 117 Plasma 118 Chalcedony 118 Agates 119 Onyx or Agate Onyx 120 Carnelian 122 Jasper 123 False Lapis 124 Hematite 124 Obsidian 125 Malachite 126 Jet 128 Amber 128 Coral 130

Table of Hardness and Specific Gravity 132

Index 135

PREFACE.

The object of this little book is to convey to the merchant, the workman, and the amateur, in a condensed and accurate form, information concerning the various properties of precious stones. Besides drawing freely on a number of authorities, the author has used his practical experience to indicate such tests as an amateur can readily make. Specific gravity, hardness, and dichroism are tests which are easily mastered, and a thorough understanding of these three properties will assist in classifying doubtful gems.

Such stones have been dealt with principally as are used in commerce for jewelry and ornamental purposes.

The attention of the writer has often been called to the general lack of knowledge among the jewelers regarding precious stones other than diamonds, rubies, sapphires, and emeralds.

As there are so many other beautiful and rare gems which nature yields to man, and which are worthy of the jewelers’ art, the author trusts that his book will awaken a new interest in the fascinating study of mineralogy as applied to precious stones, and that at some future day he may feel encouraged to enlarge upon this treatise.

M. D. ROTHSCHILD. 41 and 43 MAIDEN LANE, NEW YORK.

HAND-BOOK OF PRECIOUS STONES.

WHAT ARE PRECIOUS STONES?

The mineral to which the term “precious stone” is applied, must be adaptable for jewelry or ornamental purposes and must possess beauty, hardness, and rarity.

The beauty of a precious stone or gem consists of its color or colorlessness, brilliancy or softness of lustre, and transparency. To take a high and lasting polish, a mineral must be hard,—and many stones that would otherwise be highly valued are low in the estimate of worth because they do not possess of sufficient hardness to make them endure the wear and friction to which a precious stone is subjected when used in the form of jewelry. The rareness of precious stones has a decided effect in determining their values. For instance, the crocidolite, commercially known as tiger-eye, was sold by the carat some years ago, and was largely used in the making of fine jewelry. To-day, this material is so plentiful that it is no longer classed among the higher gems, but serves for cameos and intaglios like chalcedony and onyx.

The changes of fashion have much to do with determining the market value of precious stones. Amethysts, topazes, cat’s-eyes, aquamarines, alexandrites, and even emeralds and opals have been eagerly sought for at times and then again neglected for other gems, causing a sensible difference in the value of these stones.

There are all degrees of precious stones, from the valuable diamond and corundums to the humbler quartz, amethyst, and topaz.

It has been a mooted question as to the proper dividing line between stones that deserve the title “precious,” and those which should be placed in a so-called semi-precious or lower category. To draw such a line is hardly possible, as neither hardness, rareness, nor value would be a positive test—some of the hard stones, like zircon and almandines being less valuable than the softer opal, while the diamond, one of the most plentiful of precious stones, is at the same time, one of the most valuable.

Part 2

Neither can price be taken as a complete test, because fashion makes a turquois, an opal, or an emerald much more valuable at one time than at another. All precious minerals used for ornamental purposes, from the diamond to quartz, or chalcedony, may properly be termed precious stones.

PHYSICAL CHARACTERS.

CRYSTALLIZATION.

Precious stones are found either in crystallized or amorphous conditions. The forms of crystallization are:

1 Isometric or Cubic; having the axes equal. 2 Tetragonal or Pyramidal } having only the 3 Hexagonal or Rhombohedral } lateral axes equal. 4 Orthorhombic or Trimetric } 5 Monoclinic or Oblique } having the axes 6 Triclinic or Anorthic } unequal.

Most precious stones crystallize, but the specimens that have the crystallization clearly defined are seldom found. The amorphous condition includes the turquois, opal, and obsidian, which minerals are found in masses or veins surrounded by a matrix.

CLEAVAGE.

Many minerals can be separated readily in one direction by simply making a small indentation with a harder mineral, then introducing the blade of a knife into the scratch and striking it a sharp blow,—this separates the crystal. There are certain planes at right angles where the crystal can be separated; this property is called cleavage and the planes, cleavage planes.

In some minerals cleavage is difficult to produce, while in others such as mica and rock-salt, cleavage is highly perfect and the number of separations produced is only limited by the thickness of the blade used in separating the planes.

The property of cleavage is very useful and of great assistance to the lapidary, as it enables him to shape a diamond or other hard stone nearly to the size he desires, and at the same time to save the extra material cleaved off, which can be used for smaller gems, and which under other conditions would have to be ground away.

FRACTURE.

Fracture surfaces are the result of the breaking of a crystal otherwise than by cleaving, and in a different direction from the cleavage planes.

When the form of fracture is composed of concave and convex surfaces it is called conchoidal; when free from inequalities it is known as even or smooth, and when covered by small splinters, splintery or uneven.

OPTICAL PROPERTIES.

REFRACTION.

When a ray of light passes from one medium to another, or from the air to a crystal it is bent or refracted; this is called single refraction and takes place in the diamond, spinel, and garnet.

Most of the other transparent precious stones possess double refraction—that is, the ray of light enters the crystal and divides into two parts, one following the ordinary laws of refraction, while the other part or extraordinary ray does not obey the usual law.

There are precise methods for measuring the indices of refraction, but they are not applicable to polished gem stones.

POLARIZATION OF LIGHT.

Polarization is a peculiar modification which, under certain conditions, a ray of light undergoes. This property is easier to observe than double refraction.

If from a transparent prism of tourmaline two thin plates are cut, parallel to its axis, they will transmit light when they are placed above each other with the chief axis of each in the same direction.

When one of the plates is turned at right angles to the other, no light, or but very little, is transmitted, so that the plates appear black.

In passing through the first slip, the rays of light have acquired a peculiar property, which renders them incapable of being transmitted through the second, except when the two are held in a parallel position, and the rays are then said to be polarized.

In some doubly refracting crystals the two oppositely polarized beams are of different colors, so upon double refraction and polarization depends the property of many gems which is called pleiochroism.

PLEIOCHROISM.

The dichroiscope is a handy little optical instrument, that will readily serve to distinguish the diamond, spinel, or garnet (all singly refracting minerals) from the ruby, beryl, or any of the doubly refracting stones. This instrument consists of a cleavage rhombohedron of Iceland spar, fastened in a brass tube about 2½ inches long, and ¾ of an inch in diameter. A sliding cap at one end has a perforation ⅛ of an inch square, and at the other end is a lens which will show a distinct image of the square opening when the cap is pulled out about ¼ of an inch.

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