SILICA. 239 
lina; at Pike’s Peak, Colorado, and elsewhere; amethyst at 
Bristol, R. L, and Keweenaw Point, Lake Superior ; chalce- 
duny 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 reflectrons, 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°5-6°. G.=1:9-2:3. 
Composition. Opal consists of silica, ike 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 arich 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 Paideros, 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- 
