Zircon
is a mineral remarkable among those employed as gems for its high
specific gravity and adamantine luster. For these reasons the
colorless, transparent stones are sometimes employed as substitutes
for the diamond, although they lack the high refractive power and
hence play of colors of the latter. The stones are sometimes called
"Matura diamonds," because of their abundance at Matura in the island
of Ceylon. The colorless, or smoky zircons, are often known as "
jargons" or " jargoons," a name said to have been given in allusion to
the fact that though they resembled the diamond in luster they had
really much less value. Besides zircons of this sort there are those
known in jewelry as " hyacinth " or " jacinth," which are transparent
zircons of a brownish, red-orange color. A stone of a nearly similar
color is furnished by the essonite variety of garnet, and this is also
often known as hvacinth.
The
high specific gravity of zircon above referred to is more than four
times the weight of water, determinations giving results varying
between 4.2 and 4.86. Zircon is thus the heaviest of gems, and will
sink at once in any of the ordinary heavy liquids. The hardness of
zircon is between that of quartz and topaz, being 1\. Its index
of refraction is high, being 1.92, or near to the diamond among gems, a
fact which accounts for its brilliancy when cut. Before the blowpipe
zircon is infusible. It is not acted upon by acids except in fine
powder by sulphuric acid. In composition it is a silicate of zirconium,
the percentages being silica 32.8, zirconia 67.2. It usually also
contains a little iron oxide. It is not an uncommon mineral in rocks,
occurring in crystals of microscopic size, and in crystalline rocks it
sometimes occurs in large and abundant crystals. These are usually
opaque and of no value for gem purposes, although they are mined to
some extent at the present time for use in incandescent lights. Opaque
zircon is found in this country in Georgia, Colorado, New York, and
Canada. The form of the crystals is usually that of four-sided prisms
terminated by pyramids. The transparent zircons available for gems,
that is, the so-called " noble " zircons, come almost wholly from the
island of Ceylon, where they occur in the gem gravels that contain also
rubies, sapphires,
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