The name garnet is applied to a group of six closely related silicate
minerals that are alike in crystal structure but that differ mainly in
the substitution of certain metallic elements in their composition.
These minerals are:
_Pyrope_, magnesium aluminum garnet
_Almandine_, iron aluminum garnet
_Spessartine_, manganese aluminum garnet
_Uvarovite_, calcium chromium garnet
_Grossular_, calcium aluminum garnet
_Andradite_, calcium iron garnet
Most natural garnets have compositions intermediate between members of
the basic group of six. For example, there are garnets having
compositions anywhere between pyrope and almandine, depending on the
amount of difference in the magnesium or iron content. These same
garnets may even have varying amounts of manganese, and thus be
partially spessartine.
The six garnets in the basic group are found in considerable quantity in
many areas, but seldom are they of sufficiently high quality to be
considered gemstone material. Even when stones of gem quality are found,
their colors—particularly the reds—tend to be so intense that they seem
to be opaque.
[Illustration: Garnets occur in several colors, although most people
think of them as red. Shown here are a 54-carat spessartine from
Brazil (top right), a 6-carat rhodolite from North Carolina (at
left), a magnificent 10-carat green demantoid from Russia, a 9-carat
grossular from Ceylon (bottom), and a 26-carat spessartine from
Virginia. (Seven-eighths actual size.)]
Garnet has a hardness (about 7) suitable for gemstone material and a
fairly high refractive index (1.74 and above).
Ruby red pyrope is the most popular variety of garnet. It is found in
Bohemia, in Czechoslovakia, where it occurs as small, poorly shaped
crystals. Red pyrope also is found in Africa, where it is called Cape
ruby, and in Arizona, where it is sold as Arizona ruby. Another kind of
pyrope called _rhodolite_ is noted for its soft, rosy purple color.
Actually, rhodolite is one of the intermixed garnets with a composition
somewhere between pyrope and almandine. Most of the fine rhodolite gems
have come from North Carolina.
Almandine is popular in its deep red, transparent form, but since the
red is so dark and intense that it appears black, the stones usually are
cut as cabochons with the back hollowed out. This makes them thinner,
and thus lightens their color. Garnets cut in this manner are all known
as carbuncles. Brazil, India, Ceylon, Australia, and parts of the United
States are important sources of almandine.
Although spessartine has a rich orange color, it is not often used as a
gemstone because of the relative rarity of gem-quality cutting material.
This mineral gets its name from the town of Spessart, Germany, where it
was first found. Excellent spessartine with colors ranging from orange
to brown has been found at Amelia Court House, Va., and quality gems
have been cut from such material. Ceylon, Burma, Madagascar, and Brazil
also have furnished some gem spessartine.
The chromium garnet, uvarovite, generally is too poor in quality for
cutting. Uvarovite crystals, which are emerald green in color, occur in
only small sizes. They are found mostly in Russia, Finland, and
California.
Grossular varies in color. It occurs chiefly in some shade of red,
green, yellow, or brown, depending on the impurities present. When pure,
grossular is colorless. A kind of grossular called _hessonite_ has an
attractive cinnamon color, and it is found mainly in Ceylon. Because of
its color it can easily be confused with spessartine, which it closely
resembles.
Andradite, a very common garnet, usually is found in shades of red,
black, brown, yellow, or green. Some types of gem andradite have special
names for different colors: _topazolite_, yellow; _demantoid_, green;
and _melanite_, sparkling black. The very valuable demantoid is found in
Russia and Italy.
VARIETIES:
Grossular: Colorless, green, amber, brownish yellow, rose
Hessonite: Cinnamon colored
Pyrope: Deep red
Rhodolite: Rose red and purple
Almandine: Deep red
Spessartine: Brownish red to orange
Andradite: Yellow, greenish yellow, emerald green, brownish red,
brownish yellow, brown, black
Topazolite: Yellow to greenish
Demantoid: Grass green to emerald green
Melanite: Black
Uvarovite: Green