Precious Stones, Vol. 1

by Max Bauer

Overview

This classic study begins with a general analysis of precious stones followed by descriptions of their cutting and mounting and the ways in which their value and authenticity are assessed. The remainder of this volume focuses on the diamond: its characteristics; its natural occurrences and artificial production; and its use in jewelry. 52 figures.

Product Details

ISBN-13:	9780486151250
Publisher:	Dover Publications
Publication date:	08/29/2012
Series:	Dover Jewelry and Metalwork , #1
Sold by:	Barnes & Noble
Format:	eBook
Pages:	281
File size:	10 MB

Read an Excerpt

Precious Stones

---

By Max Bauer, L. J. Spencer

Dover Publications, Inc.

Copyright © 1968 Dover Publications, Inc.
All rights reserved.
ISBN: 978-0-486-15125-0

---

CHAPTER 1

FIRST PART

GENERAL CHARACTERS OF PRECIOUS STONES

I. NATURAL CHARACTERS AND OCCURRENCE.

A. CHEMICAL COMPOSITION.

PRECIOUS stones in their general chemical relations do not differ essentially from other minerals. They are composed of the same chemical elements, which are combined together according to the same Jaws. At one time, however, it was believed that, since the characters of precious stones were so remarkable, their chemical composition must also be unique. Hence it was assumed that all precious stones contained a rare and precious earth as a fundamental constituent. More exact chemical investigations have shown, however, that the constituents of the rarest of precious stones are frequently very common substances, such as carbon or alumina. The precious metals—gold, platinum, &c.&—never enter into the composition of gems, and the rare elements very exceptionally so. As examples of such occurrence may be mentioned the element zirconium, present in zircon, and the element beryllium, present in emerald, aquamarine, and a few other rarer stones.

The chemical composition of different stones varies considerably in complexity. While it is very simple in some, in others it is complicated by the presence of numerous constituents. In the case of the diamond, the chemical composition is very simple; this, the most important of gems, consists solely of the common and widely distributed element carbon. The carbon of diamond, however, is endowed with special properties, and differs very widely from graphite, the other crystallised modification of carbon, and from coal, which consists largely of carbon. Among gems, the diamond stands alone in the simplicity of its chemical composition.

At least two, and in the majority of cases a number of, elements enter into the composition of all other precious stones. The rarest and most costly of all stones, the red ruby, contains only two elements ; and the blue sapphire is identical with the ruby in chemical composition, differing from it only in colour. The two elements of which the ruby and sapphire consist are aluminium and oxygen. The former, an important constituent of clays and other widely distributed minerals, is a metal which in recent years has become of great importance in the arts and manufactures ; the latter is an important constituent of atmospheric air. The combination of aluminium and oxygen, known as oxide of aluminium, or alumina, is an essential constituent of many other valuable gems. Rock-crystal, amethyst, agate, opal, and other stones also consist of a simple oxide, the oxide of silicon. This oxide, which is known as silica, is the most important constituent of the earth's crust. Zircon, spinel, and chrysoberyl furnish examples of slightly more complex oxides.

While the group containing the oxides furnishes so many important gems, there are other groups of minerals which are unimportant from this point of view. Such groups are those of the metallic sulphides (compounds of metals with sulphur), the haloid compounds (combinations of metals with chlorine, bromine, iodine, and fluorine), and the sulphates (compounds of sulphuric acid). Although these three groups may include minerals which are occasionally used as ornamental stones, we find none possessing the essentials of a gem to any marked degree.

The group containing the silicates is again an important one, for it embraces the emerald, garnet, chrysolite, topaz, and many other precious stones. Tourmaline may be mentioned as an example of the few gems belonging to this group, the chemical composition of which is specially complex.

Of the other divisions of the mineral kingdom there remains only to be mentioned that in which the phosphates are placed. This division contains only one gem, the turquoise. This important and valuable stone, which is composed of phosphoric acid combined with alumina and water, is remarkable, inasmuch as it is the only costly stone which contains any considerable amount of water as an essential constituent.

The ornamental stone malachite may be mentioned here as being the representative of the carbonates or compounds of carbon dioxide, and at the same time as containing a considerable amount of water as an essential constituent.

To identify any given stone and to determine the mineral species to which it belongs, a chemical analysis is often desirable and, in some cases, essential. Since this method involves the complete destruction of the substance experimented on as such, it is obviously of very limited application in the determination of precious stones of great intrinsic value. In the case of uncut stones a chemical analysis may be made of detached fragments. But with cut and polished stones, not only is a complete chemical analysis impossible, but the mere testing with acid must be avoided.

B. CRYSTALLINE FORM.

Most chemical compounds, including the majority of minerals, frequently occur as solid bodies bounded by plane faces. These shapes have been assumed on the solidification of the substance, and are due to the internal molecular forces exercised by the substance, and not to any external influence. Such definite shapes are known as crystalline forms, and substances occurring in this condition are said to be crystallised. With very few exceptions all precious stones are crystallised. Diamond, ruby, sapphire, emerald, topaz, &c., occur naturally as crystals of the finest development. Only a few, of which the most important is opal, are not bounded by the plane faces characteristic of crystallised substances, but occur only as irregularly-shaped masses. Such substances without definite external form are said to be amorphous.

Crystallised bodies, therefore, differ from amorphous bodies in that on solidification they assume a regular form bounded by plane faces, which is the outward expression of internal molecular forces. Certain peculiarities in the physical characters of crystallised bodies, which are absent in amorphous substances, are also due to these internal molecular forces. Thus it is still possible to distinguish a crystallised from an amorphous body, even though the characteristic regular boundaries of the former should happen to be absent.

The absence of the regular boundaries of a crystallised substance may be due to one or more of a variety of causes. Their free development may have been hindered by external conditions ; as, for instance, when a substance crystallises in a confined space where free development in all directions is impossible. Or, again, as often happens, in extricating a crystal from the matrix, a blow from the hammer may destroy some of the plane faces of the specimen. Moreover, in the process of cutting and polishing precious stones, the natural plane faces are always destroyed. In all these cases, however, the substance still possesses the internal structure characteristic of a crystallised body. The essential difference between a crystallised and an amorphous body lies in their internal structure, on which depends the character of the substance. The presence of plane faces in the crystallised substance is merely the outward expression of its internal structure.

A crystallised body which shows no regular boundaries is said to be crystalline or massive. When these boundaries are present the body is termed a crystal. Portions of crystalline, massive material cannot be distinguished in their external form from an amorphous substance, but their internal structure shows a very essential difference, which will be described later. A crystal, however, on account of its regular boundaries can never be confused with an amorphous body.

The knowledge of crystals and the laws governing the relations between their faces belongs to the special science of crystallography. A knowledge of the subject is essential to the correct understanding of the natural relations of minerals, including also precious stones.

It has been established that each crystallised substance, including precious stones, having a definite chemical composition, has also a crystalline form which is characteristic of the substance, or to be more correct, it may exhibit a series of crystallised forms related in such a way that each may be derived from another. Moreover, bodies of different chemical composition will in general be characterised by different crystalline forms, having, as a rule, no mutual relations.

Hence it is possible to distinguish bodies not only by their chemical composition but also by their crystalline form, and this applies equally to precious stones. It is thus obvious that a knowledge of the crystallographic relations of precious stones is not only of theoretical importance, but also of the highest practical importance, for it would enable a buyer of rough stones to distinguish a genuine from a false stone by the form alone, thus avoiding injury to the stone. This method of identification, however, is applicable only when the specimen is crystallised. In the case of massive or crystalline material, the data for its scientific determination can only be obtained from the physical characters of the specimen.

The science of crystallography is not one of which the general principles can be conveyed in a few words. Generally speaking, a complete and thorough study of the subject is necessary to obtain a knowledge of practical value. Since a detailed account of the science of crystallography is quite outside the scope of the present work, the reader must be referred to special works on the subject and to the various text-books of mineralogy, which usually contain a section devoted to crystallography. It will, therefore, be assumed in what follows that the reader possesses a knowledge of at least the elements of the subject, and is further acquainted with the elements of those sciences, such as chemistry, physics, geology, the aid of which is necessary in the study of minerals and precious stones.

It may be stated briefly here that all crystals with few exceptions can be cut by a plane into two equal parts, having the same relation between them as exists between an object and its image in a mirror. Such a plane is known as a plane of symmetry, and crystals of different substances possess different numbers of these planes. The greater the number of planes of symmetry possessed by a crystal the higher its degree of symmetry. Those crystal forms which may be cut in the same manner by the same number of planes possess the same degree of symmetry, and are grouped together into the same crystal- system. There are six of these systems, to one or other of which every mineral and every crystallised precious stone must of necessity belong. The names of the different crystal- systems with the number of planes of symmetry characteristic of each are given below:

Sometimes the symmetry exhibited by a crystal is such that only half the typical number of faces are developed. These derived forms are known as hemihedral, or half-faced forms. These forms must be distinguished from those possessing the full number of faces, which are known as holohedral, or full-faced forms. Again, from hemihedral forms may be derived, by a development of only half the faces, another group, the members of which are known as tetartohedral, or quarter-faced forms. The hemihedral and tetartohedral classes of the different systems receive special names, which, however, need not be mentioned here. All the holohedral and several of the hemihedral and tetartohedral classes are represented among precious stones.

All precious stones of the same kind, i.e., all diamonds, all emeralds, &c., exhibit forms belonging to the same crystal system ; they all possess the same degree of symmetry, and all show the same hemihedral or tetartohedral development if such is present.

It not unfrequently happens that two similarly developed crystals of one and the same mineral are so grown together as to be symmetrical with respect to each other about a certain plane, one crystal being a reflection of the other in this plane, as, for example, is shown for spinel in Fig. 60d. A regular grouping of two crystals in this way is known as a twin. Twins may generally be recognised by the presence of re-entrant angles between the faces at the edge of the plane of junction of the two crystals. Simple crystal individuals do not show such re-entrant angles. Sometimes on the second crystal of a twin a third individual may be grown in the same manner, thus giving rise to a triplet. Similarly four crystals grown together in a certain regular manner give rise to a quartet. Such regular growths are often very complex, and it is then no easy matter to discover the mutual relations of the several simple crystals.

The principal crystalline forms are too important to be ignored ; they will be described and figured below with the description of the various precious stones. To those who possess even a small acquaintance with the laws and terms of crystallography, the description of the different forms and their mutual relations will be easily intelligible; to others, however, it may present some difficulty. But as all the crystallographic details are collected together in a small space, it is open to such persons to omit them. Though their conception of a precious stone in its natural condition will, in such case, suffer, yet a fairly correct idea of the aspect and crystalline form of uncut crystallised precious stones may be obtained from an inspection of the figures.

Amorphous substances, such as opal, which are incapable of assuming a crystalline form, usually occur in irregular masses of indefinite shape, but rounded, spherical, botryoidal, reniform, or nodular masses are also found.

Crystallised bodies, and, consequently, many precious stones, are frequently not of uniform structure throughout ; they do not consist of a single crystal individual, but of several irregularly grown together. The compact mass which results from such a collection of crystalline individuals is known as a crystalline or massive aggregate. The constituent particles of such an aggregate may be of various shapes; they may be developed fairly equally in all directions, or they may be considerably elongated or shortened in one or more directions. Thus arise granular, columnar, fibrous, shelly, scaly, or other kinds of aggregates. A granular aggregate is coarsely or finely granular according to the size of the constituent particles.

Sometimes the particles are so fine that they cannot be distinguished with the naked eye, nor even with the help of a simple lens, and the mass then appears to be perfectly homogeneous. The microscope, however, reveals the fact that it is in reality an aggregate of minute grains, fibres, or scales. A truly homogeneous body appears homogeneous even under the highest powers of the microscope. A mass built up of minute particles, but with an external appearance of apparent homogeneity, is known as a compact aggregate. It often shows the rounded exterior of an amorphous body, and while its constituent particles may show regular crystal-faces the aggregate as a whole never does.

Specimens of such compact aggregates are frequently opaque, and their microscopic examination necessitates the preparation of a slice sufficiently thin to be transparent. A plate with parallel sides is cut and one side is polished. The plate is then fixed to a slip of glass with Canada-balsam, the unpolished surface being uppermost. The plate is then ground down till it is so thin as to be transparent, when the upper surface is polished. To preserve the section a glass cover-slip is cemented over it with Canada-balsam. Many important and interesting facts respecting the character of minerals and precious stones have been learnt from the microscopic examination of such thin sections, as they are called. The method has been specially useful in the examination of turquoise, chalcedony, and agate, where special difficulties lie in the way of other methods.

C. PHYSICAL CHARACTERS.

A. SPECIFIC GRAVITY.

One of the most important characters of a precious stone is its density. On this quality depends the weight of a stone of any given size. Thus of two bodies of equal size but of different material, the one having the greater density will exceed the other in weight. To give a concrete example, a cubic inch of iron weighs rather more than a quarter of a pound, while a cubic inch of oak weighs half an ounce. The cube of iron is, therefore, eight times heavier than the cube of wood.

Instead of measuring the density of a substance it is more convenient to compare the weight of any given volume with the weight of an equal volume of some standard substance. The substance usually selected as the standard is water at a temperature of 4° C. The ratio of the weight of any volume of a substance to the weight of an equal volume of water at the above temperature is known as the specific gravity of that substance. The specific gravity of a body is found, therefore, by dividing its weight by the weight of an equal volume of water. To calculate how many times one substance is denser than another the specific gravity of the former must be divided by that of the latter.

(Continues...)

---

Excerpted from Precious Stones by Max Bauer, L. J. Spencer. Copyright © 1968 Dover Publications, Inc.. Excerpted by permission of Dover Publications, Inc..
All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
Excerpts are provided by Dial-A-Book Inc. solely for the personal use of visitors to this web site.

---

Table of Contents

VOLUME I
INTRODUCTION
FIRST PART GENERAL CHARACTERS OF PRECIOUS STONES
I. NATURAL CHARACTERS AND OCCURRENCE
A. Chemical Composition
B. Crystalline Form
C. Physical Characters
(a) Specific Gravity
(b) Cleavage
(c) Hardness
(d) Optical Characters
1. Transparency
2. Lustre
3. Refraction of Light
4. Double Refraction of Light
5. Colour
6. Dichroism
7. Special Optical Appearances and Colour Effects
(e) "Thermal, Electrical, and Magnetic Characters"
1. Thermal Characters
2. Electrical Characters
3. Magnetism
D. Occurrence of Precious Stones
II. APPLICATIONS OF PRECIOUS STONES
A. Technical Applications
B. Application as Jewels
(a) Forms of Cutting
(b) Process of Cutting
(c) Boring
(d) Working on the Lathe
(e) Engraving ; Etching
(f) Colouring and Burning
(g) Mounting and Setting
(h) Faults in Precious Stones
(i) Artificial Production
(j) Counterfeiting
(k) Value and Price
III. CLASSIFICATION OF PRECIOUS STONES
SECOND PART SYSTEMATIC DESCRIPTION OF PRECIOUS STONES
Diamond
A. Characters of Diamond
1. Chemical Characters
2. Crystalline Form
3. Specific Gravity
4. Cleavage
5. Hardness
6. Optical Characters
7. Electrical and Thermal Characters
B. Occurrence of Diamond
1. India
2. Brazil
3. South Africa
4. Borneo
5. Australia
6. North America
7. British Guiana
8. Urals
9. Lapland
10. In Meteorites
C. Origin and Artificial Production of Diamond
D. Applications of Diamond
1. Application in Jewellery
2. Diamond-cutting
3. Technical Applications
4. Large and Famous Diamonds
5. Value of Diamonds
6. Imitation and Counterfeiting
VOLUME II
Corundum
Ruby
Sapphire
Other varieties
Spinel
Chrysoberyl
Cymophane
Alexandrite
Beryl
Emerald
"Precious beryl (Aquamarine, &c.)"
Euclase
Phenakite
Topaz
Zircon
Garnet Group
Hessonite
Spessartite
Almandine
"Pyrope (Bohemian garnet, "Cape ruby," Rhodolite"
Demantoid
"Grossularite, Melanite, Topazolite"
Tourmaline
Opal
Precious opal
Fire-opal
Common opal
Turquoise
Bone-turquoise
Lazulite
Callainite
Olivine
Cordierite
Idocrase
Axinite
Kyanite
Staurolite
Andalusite (Chiastolite)
Epidote
Piedmontite
Dioptase
Chrysocolla
Garnierite
Sphene
"Prehnite (Chlorastrolite, Zonochlorite)"
Thomsonite (Lintonite)
Natrolite
Hemimorphite
Calamine
Felspar Group
Amazon-stone
Sun-stone
Moon-stone
Labradorescent Felspar
Labradorite
Elæolite
Cancrinite
Lapis - lazuli
Haüynite
Sodalite
Obsidian
Moldavite
Pyroxene and Amphibole Groups
Hypersthene
Bronzite
Bastite
Diallage
Diopside
Spodumene
Rhodonite (and Lepidolite)
Nephrite
Jadeite (Chloromelanite)
Quartz
A. Crystallised Quartz
Rock-crystal
Smoky-quartz
Amethyst
Citrine
Rose-quartz
Prase
Sapphire-quartz
Quartz with enclosures
Cat's-eye
Tiger-eye
B. Compact Quartz
Hornstone
Wood-stone
Chrysoprase
Jasper
Avanturine
C. Chalcedony
Common Chalcedony
Carnelian
Plasma
Heliotrope
Agate (Onyx)
Malachite
Chessylite
"Satin-spar (Calcite, Aragonite, and Gypsum)"
Fluor-spar
Apatite
Iron-pyrites
Hæmatite
Ilmenite
Rutile
Amber
Jet
THIRD PART DETERMINATION AND DISTINGUISHING OF PRECIOUS STONES
General Methods
A. Transparent stones
Colourless stones
Greenish-blue stones
Pale blue stones
Blue stones
Violet stones
Lilac and rose-coloured stones
Red stones
Reddish-brown stones
Smoke-grey and clove-brown stones
Reddish-yellow stones
Yellowish-brown stones
Yellow stones
Yellowish-green stones
Green stones
B. Translucent and Opaque stones
"White, faintly-coloured, and grey stones"
Blue stones
Green stones
Black stones
Yellow and brown stones
"Rose-red, and lilac stones"
Stones with more than one colour
Stones with metallic lustre
C. Stones with special optical effects
APPENDIX
Pearls
Nature and Formation
Application
Pearl-fishing
Imitation
Coral
The Coral Skeleton
The Living Coral
Distribution : Coral-banks
Coral-Fishing : Application : Trade