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ECONOMIC GEOLOGY
may be worth £300 a carat; itis mostly mined from a meta-
morphic pre-Paleozoic limestone in Upper Burma.
Some varieties are named after gems of the same colour ;
thus the green is the ‘‘ oriental emerald,” the yellow the
““ oriental topaz,” and the violet the “ oriental amethyst.”
The star-ruby and star-sapphire have six rayed series of
inclusions. The impure massive form of corundum, emery,
is of great value as an abrasive. Large crystals of opaque
corundum are found in metamorphic contact zones, and are
used for the bearings in instruments, etc. The ruby and
sapphire are now manufactured by the fusion of alumina.
Emerald—The emerald—the precious variety of beryl
{(Be3Al,(SiOy)g), a beryllium aluminium silicate coloured by
chromium—is emerald green. It crystallizes in hexagonal
prisms, has the hardness of 7-5 to 8, and sp. gr. of 2:63
to 2:8. It usually occurs in pegmatites, mica-schists, and
altered limestones. Some beryls in pegmatite are among
the largest known crystals and weigh a ton each. Emeralds
in ancient times were found mostly in Upper Egypt. In
recent years the supply has come from slate in Colombia,
from mica-schist in Siberia, and from pegmatite in North
Carolina and New South Wales.
Topaz — Topaz (a fluosilicate of alumina, (AlF),Si0,
rhombic; hardness, 8; sp. gr., 3-4 to 3:6) crystallizes in
long rhombic prisms, which are traversed by basal joints.
When pure it is colourless, but it is typically yellow ; it has
been formed where gneiss, schist, and granite have been
acted upon pneumatolytically by fluoric acid. It is generally
associated with tin ores. The gems mostly come from the
Ural Mountains and Brazil.
Zircon (ZrO,, SiO,, silicate of zirconia ; hardness, 7%;
3p. gr., 4-2 t0 4-8) is second in brilliancy only to the diamond,
for which it has often been mistaken. If has been called
jargon owing to its variable colour, and varieties are known
as hyacinth and jacinth. Zircon is a common constituent of
granite and syenite, and as it is extremely resistant to weather-
ing it is one of the commonest minerals found in the heavy
residues of sands and sandstones,
Fade, a mineral of value from its translucency and coolness
to the touch, includes two species. Nephrite, the green jade
of New Zealand and Siberia, is a variety of amphibole. of
