INGENUITY AND LUXURY 



In practice it suffices to use the four hardest in the 

 original Mohs' scale. Bits of these stones are mounted 

 in the end of metal pencil-like holders which are con- 

 venient for handling. 



Testing a cut gem is a rather delicate operation, 

 as a scratch upon one of the facets might damage it 

 materially. It is customary, therefore, to scratch the 

 test-stone only upon the edge or girdle of the gem. 

 The skilled lapidary can tell by the slightest touch 

 the effect of his test-stone, so that there is little danger 

 of injury. 



Determining the specific gravity, or relative weight 

 of a stone, compared with an equal bulk of water, is 

 one of the most important steps in the process of iden- 

 tification. There are several kinds of apparatus for 

 making these tests, but perhaps the simplest and best 

 for ordinary use is a series of solutions of known spe- 

 cific gravity. The stone to be tested is placed in these 

 successively, passing from one end of the scale toward 

 the other until a solution is reached in which it just 

 floats. By having solutions that are carefully made, 

 and a sufficient number so that minute variations can 

 be detected, the exact specific gravity of any gem may 

 be determined in this manner. 



Three solutions are in general use for testing all 

 stones but those of the greatest density. Methylene 

 iodide, having a specific gravity at ordinary temper- 

 atures of 3.32, but which can be raised to 3.6 by sat- 

 urating with iodine, or lowered by the addition of 

 benzine or toluene, is an opaque liquid, having a dis- 

 agreeable odor; a solution having a gravity of 3.28 can 



[302] 



