silica. 235 



glasses, and even in ancient times was made into cups and 

 vases. Nero is said to have dashed to pieces two caps of 

 this kind on hearing of the revolt that caused his ruin, one 

 of which cost him a sum equal to $3,000. 



Amethyst. Purple or bluish-violet, and often of great 

 beauty. The color is owing to a trace of manganese oxide. 

 It was called amethyst on account of its supposed preser- 

 vative powers against intoxication. When finely and uni- 

 formly colored, highly esteemed as a gem. 



Rose Quartz. Pink or rose-colored. Seldom occurs in 

 crystals, but generally in masses much fractured, and im- 

 perfectly transparent. The color fades on exposure to the 

 light, and on this account it is little used as an ornamental 

 stone, yet is sometimes cut into cups and vases. 



False Topaz. Light yellow pellucid crystals. They are 

 often cut and set for topaz. The absence of cleavage dis- 

 tinguishes it from true topaz. The name citrine, often ap- 

 plied to this variety, alludes to its yellow color. 



Smoky Quartz. Crystals of a smoky tint ; the color is 

 sometimes so dark as to be nearly black and opaque except 

 in splinters. It is the cairngorm stone. 



Milky Quartz. Milk-white, nearly opaque, massive, and 

 of common occurrence. It has often a greasy lustre, and is 

 then called greasy quartz. 



Prase. Leek-green, massive ; resembling some shades of 

 beryl in tint, but easily distinguished by the absence of 

 cleavage and its infusibility. Supposed to be colored by a 

 trace of iron silicate. 



Aventurine Quartz. Common quartz spangled through- 

 out with scales of golden-yellow mica. It is usually trans- 

 lucent, and gray, brown, or reddish brown in color. 



Ferruginous Quartz. Opaque, and either of yellow, 

 brownish-yellow, or red color. The color is due to the 

 presence of iron oxide as an impurity, the red to the anhy- 

 drous oxide, and the brownish yellow to the hydrous oxide. 



n. CHALCEDONIC VAEIETIES. 



Chalcedony. Translucent, massive, with a glistening and 

 somewhat waxy lustre ; usually of a pale grayish, bluish, or 

 light brownish shade. Often occurs lining or filling cavities 

 in amygdaloidal rocks, and sometimes in other kinds. These 

 cavities are nothing but little caverns, into which siliceous 

 waters have filtrated at some period. The stalactites are 



