370 MINERALOGY 



mentary rocks, especially in the cavities of argillites and limestones ; 

 also as a deposit around hot springs and geysers. In the filling of 

 cavities from which other minerals or fossils have been removed 

 by solution, opal appears as pseudomorphs, especially after wood, 

 as wood opal, where even the structure of the wood fiber has been 

 replaced, step by step, and so faithfully that in some cases the 

 species of wood may be microscopically identified. 



Hyalite is a clear, colorless, botryoidal opal, having the appear- 

 ance of frogs' eggs, from which it takes its name; it is a very dense 

 opal, showing double refraction from internal stress. The best 

 specimens are obtained at Waltsch, Bohemia. 



Precious opal is a transparent to translucent variety yielding a 

 play of colors or flashes of light, as of fire, which have been attrib- 

 uted to minute fractures formed as the silica lost water. The 

 thin films of air in the cracks disperse the light, yielding the beau- 



FIG. 434. Opal Var. Hyalite from Waltsch, Bohemia. 



tiful play of colors which is so much admired in the opal as a gem. 

 The best gem opals were originally obtained in Hungary, where 

 they were found filling cracks and small cavities in an andesite. 

 More recently opals with a good play of color have been found in 

 Queensland, Australia; and a fire opal of a honey-yellow color is 

 found at Queretaro, Mexico, in a rhyolite; these opals are jelly- 

 like and lack the fire of the Austrian or Australian specimens. 

 Geyserite is an opaque white, but very porous opal, deposited 



