The mineral to which the term “precious stone” is applied, must be
adaptable for jewelry or ornamental purposes and must possess beauty,
hardness, and rarity.
The beauty of a precious stone or gem consists of its color or
colorlessness, brilliancy or softness of lustre, and transparency.
To take a high and lasting polish, a mineral must be hard,—and many
stones that would otherwise be highly valued are low in the estimate of
worth because they do not possess of sufficient hardness to make them
endure the wear and friction to which a precious stone is subjected
when used in the form of jewelry. The rareness of precious stones
has a decided effect in determining their values. For instance, the
crocidolite, commercially known as tiger-eye, was sold by the carat
some years ago, and was largely used in the making of fine jewelry.
To-day, this material is so plentiful that it is no longer classed
among the higher gems, but serves for cameos and intaglios like
chalcedony and onyx.
The changes of fashion have much to do with determining the market
value of precious stones. Amethysts, topazes, cat’s-eyes, aquamarines,
alexandrites, and even emeralds and opals have been eagerly sought for
at times and then again neglected for other gems, causing a sensible
difference in the value of these stones.
There are all degrees of precious stones, from the valuable diamond and
corundums to the humbler quartz, amethyst, and topaz.
It has been a mooted question as to the proper dividing line between
stones that deserve the title “precious,” and those which should be
placed in a so-called semi-precious or lower category. To draw such a
line is hardly possible, as neither hardness, rareness, nor value would
be a positive test—some of the hard stones, like zircon and almandines
being less valuable than the softer opal, while the diamond, one of the
most plentiful of precious stones, is at the same time, one of the most
valuable.
Neither can price be taken as a complete test, because fashion makes a
turquois, an opal, or an emerald much more valuable at one time than
at another. All precious minerals used for ornamental purposes, from
the diamond to quartz, or chalcedony, may properly be termed precious
stones.