origin
of the formation of all precious stones: and all the precious stones
known have, when analysed, been found to be almost exclusively composed
of upper-earth-crust constituents ; the other compounds which certain
stones contain may, in all cases, be traced to their matrix, or to
their geological or mineralogical situation.
In
contradistinction to this, the essentially underground liquids, with
time and pressure, form metallic minerals and mineralise the rocks,
instead of forming gems.
Thus
we see that in a different class of minerals—compounds of metals with
the sulphates, such as sulphuric acid and compounds ; also those
containing the metallic sulphides : in cases where the metalliferous
ores or the metallic elements enter into composition with the halogens
—bromine, chlorine, fluorine, and iodine—in all these, precious stones
are comparatively common, but the stones of these groups are invariably
those used for decorative or ornamental purposes, and true " gems" are
entirely absent.
It would therefore appear that though metallic minerals, as already mentioned, are formed by the action of essentially underground chemically-charged
water— combined with ages of time and long-continued pressure, rocks
and earth being transformed into metalliferous ores by the same
means—precious stones (or that portion of them ranking as jewels or
gems) must on the contrary be wholly, or almost wholly, composed of
upper-earth-crust materials, carried deep down by water, and subjected
to the action of the same time and pressure : the simpler the compound,
the more perfect and important the result, as seen in the diamond, the
ruby, and the like.
