SILICA. 23ft 



lina ; at Pike's Peak, Colorado, and elsewhere ; amethyst at 

 Bristol, R. I., and Keweenaw Point, Lake Superior ; chalce- 

 dony and agates of moderate beauty near Northampton, 

 and along the trap of the Connecticut Valley — but finer 

 near Lake Superior, upon some of the Western rivers, and 

 in Oregon ; chrysoprase occurs at Belmont's lead mine, St. 

 Lawrence County, N. Y., and a green quartz (often called 

 chrysoprase) at New Fane, Vt., along with fine drusy 

 quartz ; red jasper occurs on the banks of the Hudson at 

 Troy ; yellow jasper is found with chalcedony at Chester, 

 Mass. ; Heliotrope occupies veins in slate at Bloomingrove, 

 Orange County, N. Y. 



Switzerland, Dauphiny, Piedmont, the Carrara quarries, 

 and numerous other foreign localities furnish fine crystals. 



Opal. 



Compact and amorphous ; also in reniform and stalactitic 

 shapes ; also earthy. Presents internal reflections, often of 

 several colors in the finest varieties, exhibiting, when turned 

 in the hand, a rich play of colors of delicate shades. White, 

 yellow, red, brown, green and gray are some of the shades 

 that occur, and impure varieties are dark and opaque. 

 Lustre subvitreous. H.=5'o-6'o. G. =1*9-23. 



Composition. Opal consists of silica, like quartz ; but it 

 is silica in a different molecular state, the hardness and 

 specific gravity being less ; and, besides this, it is soluble in 

 a strong alkaline solution, especially if heated. It usually con- 

 tains a few per cent, of water — amounting in some kinds to 

 12 per cent. ; but the water is not generally regarded as an 

 essential constituent. 



VARIETIES. 



Precious Opal. External color usually milky, but within 

 there is a rich play of delicate tints. This variety forms a 

 gem of rare beauty. A large mass in the imperial cabinet 

 of Vienna weighs seventeen ounces, and is nearly as large 

 as a man's fist, but contains numerous fissures and is not 

 entirely disengaged from the matrix. This stone was well 

 known to the ancients and highly valued by them. They 

 called it P aider os, or Child Beautiful as Love. The noble 

 opal is found near Cashau in Hungary, and in Honduras, 

 South America ; also on the Faroe Islands. 



Fire Opal, Girasol. An opal with yellow and bright hya- 



