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These
boules have the same atomic structure as corundum or sapphire but no
external evidence of crystallization is found. The optic axis is
usually inclined to the length of the boule. A hexagonal outline may be
visible in cross-section. Boules of 300-400 carats are made in about
four hours. One person could attend to about 25 to 60 furnaces at a
time, but much technical skill is essential for a successful working
of the furnaces. In synthetic ruby about 7% of chromium oxide is
used—the darker variety is known as the synthetic ruby, but analysis
accounts only for half the chemical added. The colour is uniformly
distributed throughout the product. Ruby contains 0-10-1-25 of chromium
oxide.
SYNTHETIC
SAPPHIRES. When 2% of iron oxide and 1% of_titanium oxide are added to
the alumina under manufacture, synthetic sapphire is the resultant
boule. The colour is uniformly distributed—green sapphire is produced
when a mixture of the oxides of vanadium and cobalt is used. A number
of coloured varieties of synthetic gem corundum could be produced by
varying the amounts of the pigmenting agents or by mixing them, and
misleading names are given to them, e.g. synthetic topaz, danbu-rite
and kunzite. When a mixture of the pigments giving red and blue colours
are mixed, a violet sapphire is produced.
PROPERTIES
OE SYNTHETIC RUBIES AND SAPPHIRES. Chemically and physically the
synthetic and natural gems show close similarity. They show also
striking similarity in refractive index, birefringence, specific
gravity, hardness, crystallization, parting, etc.
Although
the synthetic stones are identical with the natural stones in all
essential properties, they are different in some features which are of
great value in revealing then-true nature, i.e. their artificial
character. The following are some of them: structure lines, uneven
distribution of
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