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

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.

What are Precious Stones?

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.

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.

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.

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

POLARIZATION OF LIGHT.

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.

When a stone is examined by means of the dichroiscope, it will show two images of the same hue, or of different hues, these square images (fig. 1, A) forming a right angle and being more distinct when viewed from one part of the stone than from another.

When the images are identical in color, the specimen may be a diamond, garnet, spinel, or glass. Should a red or ruby spinel approach the ruby in color, a quick and satisfactory test can be made with the dichroiscope, as the spinel will show two images of one color, while the ruby will show one image of aurora red and one of carmine red.

The dichroiscope is inexpensive, costing but a few dollars, and is very useful for rapidly deciding the species of many stones. The following is a partial list of doubly refracting stones and their twin colors.

Shades of Blue.—Lapis-lazuli, amethyst, chalcedony, spinel, zircon, sapphire, cyanite, tourmaline, turquois, odontolite, fluor spar.

Shades of Green.—Amazon stone, turquois, prase, beryl, blood-stone, epidote, emerald, malachite, chrysoprase, chrysolite, idocrase, olivine, garnet, chrysoberyl.

Shades of Yellow.—Opal, amber, topaz, beryl, jasper.

Shades of Red.—Garnet, carnelian, chalcedony, rose quartz, corundum, tourmaline, spinel, ruby.

Shades of Brown.—Zircon, garnet, smoky topaz, axinite, jasper.

Colorless.—Diamond, sapphire, spinel, zircon, topaz, rock crystal, moonstone.

Lustre.

The following is a list of some precious stones and their lustre:

Adamantine.—Diamond, zircon.

Resinous.—Garnet.

Vitreous.—Emerald, ruby, spinel.

Waxy.—Turquois.

Pearly.—Moonstone, opal.

Silky.—Crocidolite, quartz cat’s-eye.

Metallic.—Hematite.

Greasy.—Olivine.

Some stones vary in lustre, from vitreous to pearly, etc.

Streak.

It is remarkable that the streak of the diamond is gray to grayish-black, while that of the ruby is colorless or white.

Hardness.

No. 5. Apatite. Medium hardness; does not scratch glass, or only faintly; does not give out sparks against steel; easily attacked with a file.

No. 6. Felspar. Easily scratches glass; is attacked by a file, and gives some sparks against steel.

No. 7. Quartz. Quite hard; is only slightly attacked by file; gives sparks readily against steel.

No. 8. Topaz. Very hard; is not attacked by a file.

No. 9. Sapphire. Hardest of all minerals but the diamond; attacks all other minerals.

No. 10. Diamond. Attacks all minerals; is not attacked by any.

To find the hardness of a stone, begin to test with the softest mineral, so that when the number is reached which will scratch the stone, there has been no injury to the specimen under examination. Half numbers are determined by the ease or difficulty with which a stone is scratched. For example, a stone which will resist No. 7 (quartz) and which is only faintly attacked by No. 8 (topaz) may be safely put down as 7.5, while a stone which resisted No. 7 and yielded easily to No. 8 is to be classed as 7 in hardness.

These tests are readily applied to crystals or unpolished gems. With the polished stone greater care must be observed, and while a file test is often satisfactory, there is always the danger of striking the cleavage and breaking off a small piece of the stone.

Specific Gravity.

The weight of an object which is free to seek the centre of gravitation is called absolute weight, while the weight of an object compared with that of another containing the same volume of matter is called the specific weight.

If a stone weighing 16 carats is placed in a vessel filled to the brim with distilled water and the stone displaces 6 carats of water, the specific gravity of the stone would be 16 ÷ 6, or 2.66, the specific gravity of quartz.

In other words, the stone would weigh 16 carats in the air and only 10 carats in the distilled water, showing a loss of 6 carats, which is the weight of the volume of water equal in bulk to the stone;—or absolute weight, 16 carats; specific weight, 10 carats; loss, 6 carats; 16 ÷ 6 = 2.66, specific gravity.

There are several methods of ascertaining the specific gravity of a stone.

First, by placing it in liquids of known specific gravity.

Second, by weighing the stone in air and then in distilled water or alcohol, and thus learning the weight of an equal bulk of water.

Third, by measuring or weighing the water which the stone displaces when immersed in a small vessel of known capacity.

Fourth, by means of the Nicholson hydrometer, a simple instrument consisting of a hollow glass cylinder, two dishes, and a glass vessel.

As the jewelers’ balances are well adapted for the ordinary work of taking specific gravity, or can be easily adapted for such work, the second method will usually be the more practical to follow.

The author has had very satisfactory service from a $30 balance, and any well adjusted balance will give fair results.

The following accessories are necessary to take the specific gravity of a stone:

Distilled water about 60° Fahr.

A very fine thread of platinum wire with which to suspend the stone (fig. 4).

A glass-beaker for the water (fig. 3, C).

A bench to hold the beaker over the pan (fig. 2).

The distilled water is easily obtainable from any druggist. The platinum wire should be bent to hook into the top of the balance frame, (fig. 3, B) and for ordinary small stones it will be convenient to twist the other end into a cork-screw shape or receptacle (fig. 4, A).

The beaker can be a small, thin glass cup of any kind, and the bench is easily produced from wood (fig. 2) or of metal with three supports (fig. 3, A).

To ascertain the specific gravity, attach the platinum wire to the balance frame, (fig. 3, B) and allow the lower end to rest in the water; then balance this carefully by adding weights to the other side (fig. 3, D) until the balance is exact.

The stone to be weighed in water is a ruby, and weighs two carats in the air.

Clean the stone carefully with water to free it from air bubbles; then place it in the screw of the wire (fig. 4, A) and weigh carefully. If the stone weighs 1½ carats it will have displaced ½ ct. of water: or, weight in air, 2 carats; weight in water, 1½ carats; loss, ½ carat; 2 ÷ ½ = 4, which will be the specific gravity of the ruby.

The Jolly spiral balance can also be used for taking specific gravity, but it is not so practical or accurate for small stones as for the larger ones.

Weight.

The weight of the carat being arbitrary, it varies in different countries, some being heavier and others lighter than .205 milligrams.

The writer wrote to three prominent balance-makers in the United States some months ago for their carat standards and was surprised to find that they all differed. This will account for discrepancies in weight resulting between the balances of different makers. Of late there has been a decided movement in Europe, headed by the French Chambre Syndicale of jewelers, in favor of the unification of the carat, so that the weight of a French or Dutch carat will equal that of an English, American, or any other carat. This reform will probably be accompanied by the adoption of the decimal system of dividing the carat, and the discarding of the complicated fractional system.

After having tried the decimal weights for many months, the author can testify to a decided gain in time and accuracy from their use.

Fusibility.

4. Green Actinolite. Fusible before the blow-pipe in coarse splinters.

5. Orthoclase. Fusible before the blow-pipe in fine splinters.

6. Bronzite. Before the blow-pipe becomes rounded only on the sharp edges.

Magnetism.

Transparency.

The more valuable precious stones, excepting opals and turquoises, are generally transparent.

Phosphorescence.

Electricity.

Some stones lose this quality in a few minutes, while others retain it a long time. The tourmaline is noted for its electrical properties, while the Brazilian topaz rendered electric by heating or rubbing has been known to affect the electric needle after 32 hours.

Cutting and Polishing.

The brilliant cut, figs. 5 and 6, consists of an arrangement of fifty-six facets, exclusive of the table and culet. This cut is sometimes improved by the addition of eight star facets around the culet, which brings the number of facets up to sixty-four.

The following are the proportions of a well cut diamond or colorless gem:

• ⅓ above the girdle, fig.  6, A.
• ⅔ below  ""   "  6, B.
• The table ²⁄₅ of the breadth of the stone, fig. 6, C.
• The culet ⅙ of the size of the table, fig. 6, D.