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Appendix A - Discrimination of Precious Stones

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Appendix.
311
The Diamond (No. io) scratches every other stone. The Sapphire (No. 9) stands next in hardness to the Dia­mond, and scratches all inferior stones. The Topaz (No. 8) and the Rock Crystal (No. 7) are the only other minerals likely to be of service
The Opal, Turquoise, Moonstone and Sphene are all inferior to Quartz in hardness ; or, in other words, fall below No. 7 in the standard scale.
Specific Gravity.
As specific gravity is a readily applicable, yet invalu­able, aid in the discrimination of Precious Stones, a des­cription of the usual modes of taking the specific gravity of a mineral may be useful. By specific gravity is meant the relative weights of equal bulks of different kinds of of matter. Distilled water at 60 degs. F. is usually taken as the unit of comparison, so that if a gem weigh 3-1/2 times as much as an equal bulk of water, under the above con­ditions, it is said to have a specific gravity of 3'5.
One method, now much used, consists in placing the gem in a liquid of known specific gravity, and observing whether it sinks or floats. The liquid must necessarily be of very high specific gravity if it is to be of any use in dealing with gems. Such a liquid as that discovered by Mr. Sonstadt, and called after him "Sonstadt's Solution," has been used. This is a solution of mercuric iodide in potassium iodide, and can be prepared of any specific gravity up to about 3. It is, however, very poisonous, and must be used with the greatest caution. It is often known on the Continent as Thoulet's solution.
In this solution, any stone in the following list would float;—Tourmaline, Turquoise, Emerald, Beryl, all the
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