GARNET
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.
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