r embedded in
silica.

Following out the explanation of the aqueous solution, in which the
earth-crust constituents are secreted, we find that the rarer and more
precious metals do not generally enter into the composition of precious
stones--which fact may advisedly be repeated. It is, of course, to be
expected that beryllium will be found in the emerald, since it is under
the species beryl, and zirconium in zircon; but such instances are the
exception, and we may well wonder at the actions of the infinite powers
of nature, when we reflect that the rarest, costliest and most beautiful
of all precious stones are the simplest in their constituents.

Thus we find the diamond standing unique amongst all gems in being
composed of one element only--carbon--being pure crystallised carbon; a
different form from graphite, it is true, but, nevertheless, pure carbon
and nothing else. Therefore, from its chemical, as well as from its
commercial aspect, the diamond stands alone as the most important of
gems.

The next in simplicity, whilst being the most costly of all, is the
ruby, and with this may be classed the blue sapphire, seeing that their
chemical constituents are exactly the same, the difference being one of
colour only. These have two elements, oxygen and aluminium, which
important constituents appear also in other stones, but 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 in 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 ar