CATAL(JGUE OF GEMS. 519 



occasionally havino; a satin-like effect or schiller. Color, white: often 

 ora}', reddish white. Hesh red, greenish white, and occasionally oreen. 

 Transparent to translucent. 



The difference in color, luster, and other physical properties has 

 oiven rise to distinct names for several varieties of this mineral, the 

 more important of which for use as gems are: Adahiria, a transparent 

 or translucent varietj' differing from ordinary orthoclase in present- 

 ing, when polished, chatoyant or pearly reflections. The kinds exhib- 

 iting this chatoyancy to the greatest degree are known as moon.stonts^ 

 and have, as a rule, a specific gravity of 2.58. Aventurlne is less pel- 

 lucid and has reddish and yellow internal reflections arising from 

 minute scales of occluded minerals.' Siotstone is a similar variety. 

 Cassinite is a bluish-green kind, having a pearly luster and more or 

 less of an aventurine character. Leelite has a deep flesh-red color, 

 with a wax}' luster. Pert lute is a flesh-red feldspar, consisting of 

 interlaminated orthoclase and albite, and which often affords l)right 

 aventurine reflections. Variolite is a dark-green variety containing 

 lighter globular particles; it was so called in allusion to its supposed 

 power in preventing and curing smallpox. 



Orthoclase in its several varieties belongs especially to the crystal- 

 line rocks, occurring as an essential constituent in granite, gneiss, 

 syenite, porphyr}', etc. 



One hundred parts contain: Silica, Q4:.'^\ alumina, 18.4. 



PEGMATITE. t 



GRAPHIC GRANITE. 



Pegmatite consists of feldspar and quartz, in which the quartz, 

 •arranged in parallel positions, is so distributed through the feldspar 

 as to appear like oriental characters. 



PHENACITE. 



Phenacite crystallizes in the hexagonal system; has a hardness of 8; 

 a specific gravity of 2.96; it is colorless, often clouded or milky; also 

 straw and wine yellow and cinnamon colored. The clouded kinds are 

 dichroic. The colorless transparent kinds exhil)it considerable "" fire," 

 especialh' b}' artificial light, and may easily be mistaken for the dia- 

 mond, hence the name, from <Peva^^ a deceiver. 



One hundred parts contain: Silica, 54.3; glucina, 4.5.7. 



PORPHYRY. 



The term porphyry, as here used, applies to that class of rocks in 

 which the mass of the rock, or groundmass, is so compact and dense 

 as to appear practically noncrystalline, and in which are embedded 

 large, scattering, more or less perfectly formed crystals, usuallv of 

 quartz or feldspar. These crystals being of a different color from the 



NAT Mrs I'.MMi ?,^^ 



