PRECIOUS STONES. 153 



lucent lines. It is jrenerally of a blackish or brownish 

 color, stiiped with white ; occasionally, also, with a green- 

 ish layer. 



O'pal. — The jirecions opal presents a peculiar play of colors of 

 delicate tints. It consists of silex and about ten per cent, 

 of water. 



Plas'ma. — A faintly translucent chalcedony, approaching jas- 

 per, having a greenish color sprinkled with yellow and 

 whitish dots, and a glistening lustre. 



Ru'by. — A precious stone, next to the diamond in hardness 

 and value. Its constituents are aluniinn, magnesia, and 

 chromic acid, — its color being due to the latter. The rarest 

 color of this gem is pure carmine, or blood-red of consider- 

 able intensity. 



Sap'phire. — A gem having the same composition, hardness, 

 and other properties as the ruby. It varies in color from 

 white to the deepest blue and black. 



Sard. — A variety of chalcedony which has a rich brownish- 

 red color, but when held between the eye and the light 

 appears of a deep blood-red. 



Sar'donyx. — A silicious stone or gem nearly allied to onyx. It 

 is of a deep rich brown, inclining to orange, and, when 

 held against the light, exhibits a deep-red hue. 



St. Stephen's-stone. — White chalcedony, with minute bloOd- 

 red spots. 



To'paz. — A gem consisting of silica, alumina, and fluoric acid. 

 It is generally of a fine yellow or gold color. 



Tur'quoise. — A mineral of a beautiful sky-blue color; occurring 

 in thin layers or in rounded masses. It is destitute of 

 lustre, but susceptible of a high polish. It consists chiefly 

 of hydrated alumina ; and its color is probably due to 

 oxides of copper and iron. 



Yellow Sapphire. — A gem of a yellow tint, seldom deep, but 

 generally of a light straw-color, extremely brilliant. 



Zir'con, — A -crystalline mineral, transparent to translucent, of 

 various colors, adamantine lustre, and consisting of silica 

 and zirconia. It comprises three varieties, — hyacinth, 

 jargon, and zirconite. 



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