NATURE OF GEMS
Gems
are minerals prized for their color, hardness, luster, and, for the
most part, transparency. It is generally essential that a mineral to be
a gem should excel in at least three of the above-named properties,
although a few are superior in only two. Some minerals may, for
example, possess desirable color and luster, but, lacking hardness, are
little used for gem purposes, because they would become quickly marred
when worn. Fluor-spar is an illustration of such a mineral. That a high
degree of hardness is not essential, however, to the employment of a
mineral as a gem is shown by the extensive use of such substances as
pearl, amber, jet, and turquois, for gems. All of these are easily
scratched by ordinary objects. It is to be noted, however, that they
are not transparent substances, and that an opaque or translucent
substance may endure, without serious injury, scratches which would be
fatal to the beauty of a transparent gem. Hardness and color alone
cannot, however, make a mineral suitable for gem purposes. This fact is
illustrated by many varieties of corundum, which have a high degree of
hardness and good body color, but are not used for gems because not
transparent. It is evident, therefore, that no fixed rule can be
assigned for the use of a mineral as a gem, the favor or disfavor in
which it is held seeming, in many instances, to be a matter of pure
caprice.
But,
however capricious popular favor may seem to be in its estimate of the
qualities desirable in gems, it may be set down as a fairly general
rule, that the gems which combine the most of the qualities previously
mentioned are those most highly prized. Thus, a red or blue diamond,
excelling as it does all other minerals in hardness and luster, and
being the equal of any in color and transparency, is the most valuable
of gems. The ruby and sapphire excel in hardness, and have good color,
luster, and transparency. They rank among the most valuable of gems.
In
speaking of minerals which have desirable gem qualities, it must not be
supposed that this includes all occurrences of any particular mineral
species. On the contrary, only selected portions usually have the
desired qualities. A large part of the yield, even of diamond, is of no
value for gem purposes, though it all finds commercial use
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