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GARNET
This
mineral exhibits many varieties of color and of composition. The color
probably most often thought of in connection with it is dark red, but
it would be a mistake to suppose this the only color which it may
manifest. Green, red, rose, and brown are other colors which garnet
transparent enough to be used as gems exhibits, while among opaque
garnets may be found black and many varieties of the shades above
mentioned.
These
variations of color are more or less connected with differences of
composition which it may be well first of all to consider. Garnet as a
mineral is a silicate. United with silica the element most commonly
occurring is aluminum. If calcium be united with these two, the variety
of garnet known as grossularite, or essonite, or cinnamon stone, is
produced. If magnesium takes the place of calcium, then pyrope is
formed. If iron, we have almandite, and if manganese, spessartite.
Another variety of garnet, andradite, is composed of calcium and iron
in combination with silica, and still another, uvarovite, of calcium,
chromium, and silica. Though they seem to differ so much in
composition, all kinds of garnet crystallize in the same system, and
are closely allied in all their properties, so that it is an easy
matter to distinguish garnet of any variety from other minerals.
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Garnet
crystals may be of the twelve-sided form known as dodecahedrons, the
faces of which have the shape of rhombs; or the twenty-four-sided form,
known as trapezohedrons, the faces of which have the shape of
trapeziums. Quite as commonly occur crystals which are combinations of
these two forms, and then exhibit thirty-six faces, as in the crystal
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