Chapter VIII

PHYSICAL PROPERTIES.

F—Specific Gravity.

The fixing of the specific gravity of a stone also determines its group position with regard to weight; its colour and other characteristics defining the actual stone. This is a safe and very common method of proving a stone, since its specific gravity does not vary more than a point or so in different specimens of the same stone. There are several ways of arriving at this, such as by weighing in balances in the usual manner, by displacement, and by immersion in liquids the specific gravity of which are known. Cork is of less specific gravity than water, therefore it floats on the surface of that liquid, whereas iron, being heavier, sinks. So that by changing the liquid to one lighter than cork, the cork will sink in it as does iron in water; in the second instance, if we change the liquid to one heavier than iron, the iron will float on it as does cork on water, and exactly as an ordinary flat-iron will float on quicksilver, bobbing up and down like a cork in a tumbler of water. If, therefore, solutions of known but varying densities are compounded, it is possible to tell almost to exactitude the specific gravity of any stone dropped into them, by the position they assume. Thus, if we take a solution of pure methylene iodide, which has a specific gravity of 3.2981, and into this drop a few stones selected indiscriminately, the effect will be curious: first, some will sink plump to the bottom like lead; second, some will fall so far quickly, then remain for a considerable time fairly stationary; third, some will sink very slowly; fourth, some will be partially immersed, that is, a portion of their substance being above the surface of the liquid and a portion covered by it; fifth, some will float on the surface without any apparent immersion. In the first case, the stones will be much heavier than 3.2981; in the second, the stones will be about 3.50; in the third and fourth instances, the stones will be about the same specific gravity as the liquid, whilst in the fifth, they will be much lighter, and thus a rough but tolerably accurate isolation may be made.

On certain stones being extracted and placed in other liquids of lighter or denser specific gravity, as the case may be, their proper classification may easily be arrived at, and if the results are checked by actual weight, in a specific gravity balance, they will be found to be fairly accurate. The solution commonly used for the heaviest stones is a mixture of nitrate of thallium and nitrate of silver. This double nitrate has a specific gravity of 4.7963, therefore such a stone as zircon, which is the heaviest known, will float in it. For use, the mixture should be slightly warmed till it runs thin and clear; this is necessary, because at 60° (taking this as ordinary atmospheric temperature) it is a stiff mass. A lighter liquid is a mixture of iodide of mercury in iodide of potassium, but this is such an extremely corrosive and dangerous mixture, that the more common solution is one in which methylene iodide is saturated with a mixture of iodoform until it shows a specific gravity of 3.601; and by using the methylene iodide alone, in its pure state, it having a specific gravity of 3.2981, the stones to that weight can be isolated, and by diluting this with benzole, its weight can be brought down to that of the benzole itself, as in the case of Sonstadt's solution. This solution, in full standard strength, has a specific gravity of 3.1789, but may be weakened by the addition of distilled water in varying proportions till the weight becomes almost that of water.

Knowing the specific gravity of all stones, and dividing them into six groups, by taking a series of standard solutions selected from one or other of the above, and of known specific gravity, we can judge with accuracy if any stone is what it is supposed to be, and classify it correctly by its mere floating or sinking when placed in these liquids. Beginning then with the pure double nitrate of silver and thallium, this will isolate the stones of less specific gravity than 4.7963, and taking the lighter solutions and standardising them, we may get seven solutions which will isolate the stones as follows:—

Ashows thestones which havea specific gravity over4.7963

B"""3.70and under4.7963

C"""3.50"3.70

D"""3.00"3.50

E"""2.50"3.00

F"""2.00"2.50

G""——under2.00

Therefore each liquid will isolate the stones in its own group by compelling them to float on its surface; commencing with the heaviest and giving to the groups the same letters as the liquids, it is seen that—

Group A.—Isolates gems with a specific gravity of 4.7963 and over 4.70; in this group is placed zircon, with a specific gravity of from 4.70 to 4.88.

Group B.—Stones whose specific gravity lies between 3.70 and under 4.7963.

Garnets,many varieties. See Group D below.

Almandine4.11and occasionally to4.25

Ruby4.073"4.080

Sapphire4.049"4.060

Corundum3.90"4.16

Cape Ruby3.861

Demantoid3.815

Staurolite3.735

Malachite3.710and occasionally to3.996

Group C.—Stones whose specific gravity lies between 3.50 and under 3.70.

Pyrope (average)3.682

Chrysoberyl3.689and occasionallyto 3.752

Spinel3.614"3.654

Kyanite3.609"3.688

Hessonite3.603"3.651

Diamond3.502"3.564

Topaz3.500"3.520

Group D.—Stones whose specific gravity lies between 3 and under 3.50.

Rhodonite3.413and occasionally to3.617

Garnets3.400"4.500

Epidote3.360"3.480

Sphene3.348and occasionally to3.420

Idocrase3.346"3.410

Olivine3.334"3.368

Chrysolite3.316"3.528

Jade3.300"3.381

Jadeite3.299

Axinite3.295

Dioptase3.289

Diopside2.279

Tourmaline (yellow)3.210

Andalusite3.204

Apatite3.190

Tourmaline (Blue and Violet)3.160

Tourmaline (Green)3.148

" (Red)3.100

Spodumene3.130and occasionally to3.200

Euclase3.090

Fluorspar3.031and occasionally to3.200

Tourmaline (Colourless)3.029

Tourmaline (Blush Rose)3.024

Tourmaline (Black)3.024and occasionally to3.300

Nephrite3.019

Group E.—Stones whose specific gravity lies between 2.50 and under 3.000.

Phenakite2.965

Turquoise2.800

Beryl2.709 and occasionally to 2.81

Aquamarine2.701"2.80

Labradorite2.700

Emerald2.690

Quartz2.670

Chrysoprase2.670

Jasper2.668

Amethyst2.661

Hornstone2.658

Citrine2.658

Cordierite2.641

Agate2.610

Chalcedony2.598 and occasionally to 2.610

Adularia2.567

Rock-crystal2.521 and occasionally to 2.795

Group F.—Stones whose specific gravity lies between 2.00 and under 2.50.

Haüynite2.470and occasionally to2.491

Lapis lazuli2.461

Moldavite2.354

Opal2.160and according to variety to2.283

" (Fire Opal)2.210(average)

Group G.—Stones whose specific gravity is under 2.00.

Jet1.348

Amber1.000

(See also list of stones, arranged in their respective colours, in Chapter XII.)

In many of these cases the specific gravity varies from .11 to .20, but the above are the average figures obtained from a number of samples specially and separately weighed. In some instances this difference may cause a slight overlapping of the groups, as in group C, where the chrysoberyl may weigh from 3.689 to 3.752, thus bringing the heavier varieties of the stone into group B, but in all cases where overlapping occurs, the colour, form, and the self-evident character of the stone are in themselves sufficient for classification, the specific gravity proving genuineness. This is especially appreciated when it is remembered that so far science has been unable (except in very rare instances of no importance) to manufacture any stone of the same colour as the genuine and at the same time of the same specific gravity. Either the colour and characteristics suffer in obtaining the required weight or density, or if the colour and other properties of an artificial stone are made closely to resemble the real, then the specific gravity is so greatly different, either more or less, as at once to stamp the jewel as false. In the very few exceptions where chemically-made gems even approach the real in hardness, colour, specific gravity, &c., they cost so much to obtain and the difficulties of production are so great that they become mere chemical curiosities, far more costly than the real gems. Further, they are so much subject to chemical action, and are so susceptible to their surroundings, that their purity and stability cannot be maintained for long even if kept airtight; consequently these ultra-perfect "imitations" are of no commercial value whatever as jewels, even though they may successfully withstand two or three tests.