GEMS, NATURAL AND ARTIFICIAL 



be made of cadmium borotungstate, and reduced by 

 the addition of water to any desired lower density; 

 and a liquid known as Sonstadt's solution, which can 

 be made up in solutions of varying density, is an 

 aqueous solution of potassium iodide and mercuric 

 iodide, and is a deadly corrosive poison. 



Any of these solutions will do for testing the lighter 

 gems; and even the diamond may be tested with the 

 methylene iodide saturated with iodine or iodoform. 

 But for the heavier stones a colorless compound of 

 the double nitrate of silver and thallium, a substance 

 discovered by the Dutch mineralogist Retgers, which 

 melts at a fairly low temperature, having a specific 

 gravity of 5., and which may be reduced to any desired 

 density by the addition of warm water, is used. As 

 no gem has a density as high as 5., Retger's com- 

 pound may be used for determining the specific gravity 

 of those stones that are too heavy for testing in the 

 other solutions. 



The following list gives the specific gravity of some 

 of the principal gem stones: 



Jargoon 4.7 Chrysolite 3 .3 to 3 .5 



Garnet 4.2 Peridot 3 .3 to 3 . 5 



Ruby 3 .9 to 4 . 2 Kunzite 3.2 



Asteria 3 .9 to 4.2 Tourmaline .... 2 .9 to 3 .3 



Sapphire 3 .9 to 4 .2 Phenakite 2.9 



Diamond 3 .52 Turquoise 2 .6 to 2 .8 



Chrysoberyl 3 . 5 to 3 . 8 Emerald 2 . 6 to 2 . 7 



Alexandrite 3 . 5 to 3 . 8 Amethyst 2 . 5 to 2 . 8 



Cat's-eye 3 . 5 to 3 . 8 Moonstone 2 . 39 



Spinel 3 5 to 3 . 6 Opal 2.21 



Topaz 3.4103.6 



The action of light upon precious stones, the opti- 

 cal properties known as refraction, dispersion, polari- 

 zation, and pleochroism, furnish means of identi- 



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