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the
position of the principal crystal and optical directions in the boules
are not constant. The lapidaries also do not orientate them properly to
show dichroism on the table. On the other hand, the natural stones are
well orientated and hence not dichroic when examined in the same way.
This difference is of great help to distinguish artificial from natural
cut stones.
The
manufacture of synthetic gems on a commercial scale did not begin until
after the beginning of the present century. So rubies and sapphires
antedating 1900 are in all probability natural. No synthetic star
stones have been manufactured. All star rubies and sapphires must be
natural stones.
SYNTHETIC
SPINEL. It is manufactured by the Verneuil process. Beautiful spinels
were first produced in 1926, and now synthetic spinels with a variety
of colours are on the market. The process is the same as in corundum.
Evidence of crystalline structure is present as also cubical cleavage,
hardness, specific gravity, etc. Since it is isotropic, it does not
show dichroism, which distinguishes it from synthetic or natural
corundum and natural zircon and chrysoberyl.
The value and uses of synthetic gems.—The
natural and synthetic rubies and sapphires are so much alike in all
essentials, the artificial stones are in great demand; their prices are
only (Rs.6 to Rs.10) $ 2-3 per carat depending upon the quality,
whereas the natural stones of the same quality and weight are much more
expensive. A large demand has also arisen for watch jewels in place of
natural stones, in physical and scientific apparatus, where hard
bearing surfaces are required. They are manufactured in Switzerland,
France and Germany with a combined daily output of about a million
carats.
IMITATION GEMS. They are made mostly of glass, but some are made of plastics and some of bakelite. The
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