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consists
of glass and the lower portion some mineral or glass separated by a
piece of foil, a coloured layer or a thin plate of coloured mineral to
give the stone a desirable tint. By immersing in liquids with proper
refractive index, the deception could be easily detected.
Chapter X
PRECIOUS METALS FOR MOUNTING GEM-STONES
GOLD,
platinum and silver are widely used for mounting gems, but gold and
platinum are the more important as they do not tarnish as silver. As
pure gold is very soft and malleable, for a durable mounting, it is
usually alloyed with. other metals to increase its hardness. The gold
content of the alloy is expressed in carats or l/24th parts. Usually,
18 carat gold containing 18 parts gold and 6 parts alloy has good
wearing qualities. The usual alloys are:— yellow gold: 18 parts gold, 3
parts silver, and 3 parts copper by weight (darker shades are obtained
by increasing copper content); white gold: 75% gold, 17% nickel, 2.5%
copper, and 5.5% zinc (this alloy is harder and more durable than
platinum and iridium and serves as a substitute); green gold: 75% gold,
22.5% silver, 1.5% nickel and 1% copper (it is also used for mounting
purposes).
Pure
platinum is soft and flexible and has to be alloyed with a metal to
give necessary hardness and rigidity. Alloyed with 10% iridium,
hardness of platinum is considerably increased. This is also suitable
for mounting gems. Silver is not used for mounting of gems.
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