ll the garnets are silicates and yet we have these four principal
mineral species, which, however, are more closely related to each other
in crystal form, in character of composition and in general properties,
than is usual among the other silicates. Specimens which have any one of
the four types of composition unblended with any of the other types
would be found to be exactly alike in properties. As was suggested
above, however, there is a great tendency to blend and this is well
illustrated by the magnificent _rhodolite_ garnets, of rhododendron hue
which were found in Macon County, North Carolina. These had a
composition between almandite and pyrope, that is, they had both
magnesium and iron with aluminum and silica.

The true TOPAZ next calls for consideration as it too is a silicate. The
metallic part consists of aluminum, and there are present also the
non-metals fluorine and hydrogen. Here we have five elements in the one
substance. Various specimens of this species may be wine yellow, light
blue, or bluish green, pink or colorless, yet they all have essentially
the same properties.

TOURMALINE is about as complicated a mineral as we have. It is a very
complex silicate, containing aluminum, magnesium, sodium (or other
alkali metal, as, for example, lithium), iron, boron, and hydrogen. As
Ruskin says of it in his _The Ethics of the Dust_, when Mary asks "and
what is it made of?" "A little of everything; there's always flint
(silica) and clay (alumina) and magnesia in it and the black is iron,
according to its fancy; and there's boracic acid, if you know what that
is: and if you don't, I cannot tell you to-day and it doesn't signify;
and there's potash and soda; and on the whole, the chemistry of it is
more like a mediaeval doctor's prescription, than the making of a
respectable mineral." The various tourmalines very closely resemble each
other in their properties, the slight differences corresponding to
differences in composition do not alter the general nature of the
material.

MOONSTONE belongs to a species of mineral known as feldspar. The
particular feldspar that furnishes most of the moonstone is orthoclase,
a silicate of potassium and aluminum. Another feldspar sometimes seen as
a semi-precious stone is _Labradorite_. _Amazonite_, also, is a
feldspar. _Sunstone_ is a feldspar which includes tiny flakes or
spangles of some other mineral.

The mineral species _olivine_ gives us _peridot_. It is a silicate