CATALOGUE OF GEMS. 



521 



QUARTZ. 



Quartz crystallizes in the hexag-onal S3'steni, the most common form 

 beino- a six-sided prism, oenerally striated horizontally. The crystals 

 are often highly moditied; some appear to have each alternate plane 

 of the prism suppressed; some have the prism with p3'ramids at each 

 end; others have the two pyramids only; still others have the edges 

 and angles replaced hy new planes. It occurs also in stalactitic, mam- 



millary, and other imitative shapes, and massive, either impalpable, 

 fine, or coarse granular. Hardness, 7; specific gravity, 2.5 to 2.8, 

 depending- upon the amount of impurity present, the purest kinds 

 2.65. Luster, vitreous, sometimes inclining- to resinous, and varying- 

 in degree from splendent to dull. Colorless when pure; often various 

 shades of yellow, red, brown, green, blue, and ))lack. Quartz is 

 doubly refractive and the colored kinds are dichroic. (Plate 5.) 



