PHYSIOGRAPHY. 89 



Opal. 



Hardness =5-5 . . . 6-5. Sp. gr. =2-091, a milk-white 

 variety ; =2'060, a brownish red variety. 



Compound Varieties. Small reniform, botryoidal, and 

 stalactitic shapes, and'large tuberose concretions : surface 

 of the former smooth, of the latter rough, composition im- 

 palpable, fracture conchoidal. Massive, composition im- 

 palpable ; fracture conchoidal, even. Pseudomorphoses of 

 Calcareous Spar. 



1. The present species has been greatly subdivided into varieties, and 

 even treated of under several different species. Hyalife (amiatite) in- 

 cluded the small reniform, botryoidal, and sometimes stalactitic shapes, 

 white, and generally transparent ; Menilite, the large tuberose forms, of 

 a brownish grey color, and opake ; Precious Opal, the varieties which 

 exhibit the play of colors ; Wood- Opal, those which appear in the shape 

 of trunks, branches, and roots of trees ; Common Opal, and Semi- Opal, 

 consist of those varieties whicli have a conchoidal fracture, with medium 

 degrees of transparency and lustre; Hydrophane, of an opake, dull va- 

 riety, which becomes translucent on being immersed in water or some 

 transparent fluid. Siliceous Sinter is a deposit from hot springs, &c., 

 and according to its specific gravity belongs to the present species. 



2. Before the blow-pipe, water is disengaged, the mineral decrepitates 

 and becomes opake, showing the properties of pure silica. Two pieces 

 rubbed together, give a phosphorescent light, like Quartz. 



3. Analysis. 

 By BUCHOLZ. By KLAPROTH. 



var. Hyalite. var. Pnvioys Opal. var. Menilite. 



Silica - 92-00 - 90-00 - - 85 50 



Water - G-33 - - 10-00 - - 11-00 



The Menilite, like several other varieties, contains a small proportion 

 of oxide of iron, alumina, lime and carbon. One variety, called Opal- 

 Jasper, contains 47 p. c. of oxide of iron. The contents of water are 

 considered foreign to the mixture of the mineral, and are supposed to 

 change with the hygrometric state of the atmosphere. 



4. Opal forms short, irregular veins, generally in porphyry. It often 

 accompanies Calcedony in the vesicular cavities of amygdaloidal rocks, 

 and even in agate balls. Menilite is found in adhesive clay-slate. Some 



