68 NEW YORK STATE MUSEUM 



B CRYPTOCRYSTALLINE VARIETIES 



Chalcedony. Manimillary. Uniform in tint 

 Carnelian. A clear, red chalcedony 

 Chrysoprase. Apple green. Color due to nickel 

 Prase. Dull, leek-green 



Agate. A variegated chalcedony. Colors are banded, irregu- 

 larly clouded or in niosslike dendritic forms 

 Onyx. Parallel layers light and dark 

 Jasper. Impure, opaque. 



Quartz occurs as a constituent of many rocks such as granite, 

 gneiss, quartz porphyry, syenite, sandstone, etc. and as a vein 

 mineral in rocks of all geologic horizons. Its distribution is so 

 extensive as to preclude its limitation to any given area. Quartz 

 rocks are extensively used for building stone; chalcedonic 

 varieties are often polished for ornamental objects and massive 

 varieties are ground and used in the manufacture of sandpaper, 

 glass and porcelain and as an acid flux in some metallurgic 

 processes. 



Opal Si0 2 .nH 2 



Like quartz, opal is composed of silica or silicon dioxid, but 

 contains from 5$ to 12$ water. 



Opal shows no evidences of crystallization and is therefore 

 considered amorphous. It occurs in transparent to translucent 

 milky white or red masses and veins, often characterized by 

 internal reflections and rich play of colors; in waxy masses 

 yellow, red, brown, green, gray or blue in color; in opaque, 

 porous, brittle stalactitic masses deposited by geysers and hot 

 springs and in earthy varieties. 



VARIETIES 



Precious opal. Exhibits play of color. Used as a gem 



Fire opal. Red, firelike reflections 



Common or semiopal. In part translucent with greasy luster 



Wood opal. Pseudomorphous after wood 



Hyalite. Colorless, transparent, droplike masses 



Oeyserite. Porous opal, deposited from hot water carrying 



silica 

 Tripolite. Massive, chalklike silica composed of the remains of 



diotomes. 



