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A good method of distinguishing stones is by ascertaining their specific gravities since each variety of gem stone possesses a gravity which is very nearly constant. Reference to tables, such as the one given in the Appendix, will either immediately show what the
stone is or limit its identification very considerably. If it were possible to take gem stones all of the same size and weigh them, it would be found that the weights would be different. In other words, their weights will vary according to their denseness, and if these weights can be compared, each stone of the same volume being used, a useful constant can be obtained by which a stone may be distinguished.

By specific gravity we mean the weight of any solid compared with the weight of an equal volume of water at 40 C. At this temperature, the weight of one cubic centimeter of water is one gram, and this is the standard with which all solids are compared. For instance, one cubic centimeter (1 c.c) of carnelian would weigh 2.66 grams, and the specific gravity (sp.g.) of carnelian is therefore designated as 2.66.

There are several methods of obtaining the specific gravity of a stone, some being only suitable where laboratory apparatus is at hand. A quick method, which is only comparative but still very useful, depends on the fact that a solid will sink in a liquid having a lighter density than itself, and will float if its specific gravity is lighter than the liquid. If the density of the liquid is first known, a good idea as to the specific gravity of the stone will be obtained. Since stones are relatively heavy, solutions having a high density must be used. Those most frequently chosen are:
1.    Bromoform. Sp.g. 2.90. This is a yellowish liquid, which will mix with benzine, toluene, and alcohol, and it can thus be diluted with any of these lighter liquids to alter its specific gravity. It is fairly cheap to use.
2.    Methylene iodide. Sp.g. 3.32 A liquid which will also mix with the above named lighter liquids. It is much more expensive (ban bromoform.
3.    Klein’s solution. This is a saturated solution of cadmium-boro-tungstate with water, with a sp.g. of 3.28 at 150 C.
4.    Sonstadt’s solution. A saturated solution of potassium mercuric iodide in water, sp.g. at 150 C being 3.18.
5.    Clerici’s solution. This is colorless, having a sp.g. of 4.15, I)iit its density may be lowered by adding water. It contains thallium salts and is poisonous.
6. Retger’s solution. A double nitrate of thallium and silver, sp.g. at 150 C being 4.6.