Zircon
is recovered from the Travancore beach sands by the Travancore Minerals
Co., Ltd., and is exported to U.K., Germany, U.S.A. and Austria for
industrial purposes. The sands here contain 6% of zircon. It occurs
only rarely in large amounts. The largest crystal on record weighed 15
lb. which is exceptional.
Chapter XX
OLIVINE OR PERIDOT
Introduction
IT
is an important rock-forming mineral and is widely known. Olivine is
known to the jewellers as 'peridote' and 'chrysolite'. The term olivine
is also improperly used for green garnet and other green stones. It
crystallizes in the orthorhombic system and may occur as crystals or
grains. The cleavage is pinacoidal in two directions. Fracture is
conchoidal. Specific gravity = 3-2 to 3-4 or even higher. Hardness =
6-1/2 to 7 (not high enough for a gem). Colour: Various shades of
green, yellow, brown, reddish, etc. The best colour is a beautiful
bottle green. Peridote is olive-green. Lustre: Vitreous to greasy;
transparent to translucent. Olivine is biaxial and optically positive.
Mean refractive index = 1-66 to 1-68. Double refraction = 0-04 (high).
The dispersion is 0-018. Composition: Mg2SiO4-Fe2Si04. Intermediate varieties are called olivine.
Uses
Peridote
has a bottle green colour and is used for gems. Its hardness is 6, and
being less than that of quartz, the mineral is subject to quick wear.
Hence it is not suitable for rings. The stones are usually cut
brilliant or step, hut the cabochon is sometimes used.
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