this example is sufficient to prove their simplicity of origin.
Another
unique stone is the turquoise, in that it is the only rare gem
essentially containing a great proportion of water, which renders it
easily liable to destruction, as we shall see later. It is a
combination of alumina, water, and phosphoric acid, and is also unique
iu being the only known valuable stone containing a phosphate.
Turning
to the silica series, we again find a number of gems with two elements
only, silica—an important constituent of the earth's crust—and
oxygen—an important constituent of atmospheric air. In this group may
be mentioned the opal, amethyst, agate, rock-crystal, and the like, as
the best known examples, whilst oxygen appears also mostly in the form
of oxides, in chrysoberyl, spinel, and the like. This silica group is
extremely interesting, for in it, with the exception of the tourmaline
and a few others, the composition of the gems is very simple, and we
find in this group such stones as the chrysolite, several varieties of
topaz, the garnet, emerald, etc., etc.
Malachite
and similar stones are more ornamental than precious, though they come
iu the category of precious stones. These are the carbonate series,
containing much carbonic acid, and, as may be expected, a
considerable proportion of water in their composition, which water
can, of course, be dispelled by the application of heat, but to the
destruction of the stone.
From
all this will be seen how strong is the theory of aqueous percolation,
for, given time and pressure, water charged with earth-crust
constituents appears to be the
