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Numbers 1 and 2 are the most convenient to use, but any or all can be made up and kept in glass test tubes ready for use. It will be realized that all varieties of quartz, whose specific gravity is about 2.66, will float on bromoform; many similarly colored stones will sink. Some of the liquids are poisonous to handle, others are unstable, so certain precautions must be taken in their use.

Another method which involves the use of an ordinary jeweler’s balance and very little other apparatus will determine the specific gravity of a stone fairly accurately. This method is based on the well known principle that, when a solid is totally immersed in water, the apparent loss in weight is equal to the weight of water displaced. The stone must first be weighed, and then weighed again while totally submerged in water. The difference between these two weights, the apparent loss, is the weight of water displaced. This figure, divided into the weight in air, gives the specific gravity of the stone.

Any delicate balance can be easily adapted. For weighing in water, a piece of thin wire coiled to act as a holder or cage for the stone may be used. This can be suspended from one arm of the balance so that the cage reaches into a small beaker of water. The beaker must stand free of the balance, and the weight of the wire must also be ascertained.
For instance, the following figures will be given as an example, carat weights being used.

Weight of wire.    .80 carats.
Weight of wire and stone.    13.65 carats.
Weight of stone.    13.65 - .80 =    = 12.85 carats.
Weight of wire partly immersed.    .72 carats.
Weight of wire and stone partly immersed.    10.38 carats.
Weight of stone in water.    10.38 — .72 =    = 9.66 carats.
Loss of weight of stone in water.    12.85-9.66    = 3.19 carats.
Specific gravity of stone.    12.85
= 4.02.
3-i9

If desired, a counterpoise of the same weight as the wire may be used, and if another of the same weight as the scale pan on the side from which the cage is suspended is used, this scale pan may be dispensed with altogether. Weighing in water will thus be facilitated.
In order to obtain more accurate results, certain additional measures should be taken. Distilled water at a temperature of 60 ° F should be used, and the weight of the atmosphere might be taken into account. Surface tension also produces small errors; the high surface tension of water, which makes it cling to the wire cage and also causes bubbles, makes results inaccurate. In order to avoid this, other liquids are sometimes used. Alcohol, toluol (sp.g. .87), carbon tetra-chloride (sp.g. 1.59), and ethylene dibro-mide (sp.g. 2.18) are among these, and at the laboratory of the Precious Stone Section of the London Chamber of Commerce, Anderson and Payne have recommended the use of the last. When using any of these liquids, it is necessary to allow for the density of the liquid at the temperature when the experiment is being carried out. Tables showing details of such necessary corrections may be obtained. But for ordinary practical purposes, the method first described should give sufficiently accurate results.

Specific gravity may be ascertained without damaging most stones since very few are affected by water. Turquoise, opal, and porous stones should not be tested in liquids other than water. There are balances which have been specially devised for taking specific gravities, and among these are the aerometer, Walker’s balance, and Jolly’s spring balance. For small fragments or powdered minerals, a specific gravity bottle may be used. Reference to these is made in a later chapter.

In general, crystalline solids expand on being heated, and the amount of expansion is different in different directions. The expansion in volume is equal to the sum of the linear expansion along the principal axes, thus showing that the crystal form of a stone will have some bearing on the heat effects. But this is a matter for those who can experiment in laboratories. Here wc will note the effect of heat on gem stones in general, for in recent years, attempts to alter the color of certain varieties by heating have been on the increase. Some success has been obtained, mostly at the expense of experiment. The theoretical causes of many of such changes are not yet fully understood.

In some instances, it may be the result of a re-arrangement of molecular structure; in others, some chemical change may take place. For instance, if the color is due to a minute portion of included organic matter, heat would permanently destroy the original color. If the color is due to inorganic matter, such as a metallic oxide, the original color may return on cooling, or it may be changed altogether. But the general effect of heat is to discolor stones.

If a very high temperature is reached, most gem stones are completely spoiled. Those which develop pyro-electricity have already been noted. Others, which change color when certain temperatures are reached, are discussed under their individual headings. For instance, sherry colored topaz loses its color completely on being heated, yet on cooling it changes to an attractive pink. Many of the pink topaz on the market have been artificially “pinked” by packing in magnesia, charcoal, or Plaster of Paris, and then being slowly heated. Incidentally, this is a practice seldom carried out now with topaz, and most of the heat treated stones are found in old mounted jewelry. The stones are often foiled to give extra depth of color.