Moonstone,
according to the mineralogical concepts of the United States National
Museum, is a transparent albite having a chatoyant reflection
resembling that of a cat's-eye, or an opaque pearly white albite having
a bluish opalescence. Albite occurs in opaque to transparent masses
and in triclinic crystals having a dual cleavage in different
directions, one of which is highly perfect; hardness, 6; specific
gravity, 2.62; lustre vitreous, sometimes pearly on a cleavage surface;
colours, white, bluish, greyish, reddish, greenish, and green, with,
occasionally, a bluish chatoyancy or play of colour. One hundred parts
of albite contain: silica, 68.7; alumina, 19.5; soda, 11.8.
Albite
is a constituent of many crystalline rocks, -anti-frequently repfctces
feldspar as a constituent of granite, of syenite, and of greenstone;
sometimes it is associated with feldspar and dolomite. Common
occurrences are in veins or cavities in granite or granitoid rocks,
which are also sometimes repositories of fine crystals of other gem
minerals, such as beryl, tourmaline, and smoky quartz.
The
moonstone of commerce comes chiefly from Ceylon, where it is found in
pieces several inches in diameter resulting from the decomposi-
