Garnet

Garnet, group of related minerals, often used as gemstones or abrasives. Large quantities of garnets are ground up and made into a variety of sandpaper. Garnets crystallize in the isometric system, usually as rhombic dodecahedrons (see Crystal). The different varieties of garnet exhibit almost all colors except blue. Brown, red, green, yellow, black, and colorless stones are common. Darker stones are usually opaque, and light ones may be transparent or translucent. The hardness of garnet varies from 6 to 7.5, and the specific gravities, or relative densities, of specimens range from 3.6 to 4.3. Garnets have a vitreous or resinous luster, and some varieties exhibit considerable brilliance. Chemically, garnets are compound silicates. However, the composition of individual specimens varies widely, and the formulas given below are only approximate.

Grossularite, Ca₃Al₂(SiO₄)₃, is a light-colored or colorless garnet, usually found in shades of green, red, yellow, or brown. It occurs in crystalline limestones, gravels, and sands in Sri Lanka, Madagascar, Brazil, Canada, Russia, and the United States. Yellow gems of this variety are often called hyacinths, and yellow and cinnamon-brown specimens are marketed under the names hessonite and cinnamon stone.

Pyrope, Mg₃Al₂(SiO₄)₃, is the variety of garnet most often used for gem purposes and is prized for its ruby red color. It occurs in igneous rock, particularly peridotite, and in alluvial deposits found in Arizona, South Africa, Argentina, Australia, Brazil, and Switzerland. Pure pyrope has no color, but all specimens contain impurities that produce shades from red to black.

Spessartite, Mn₃Al₂(SiO₄)₃, is not a popular gemstone because of its color, which is usually brownish, although it occasionally has a reddish cast. It is found in pegatites and alluvial deposits in the Alps, in Sri Lanka, and in Nevada and Virginia.

Almandine, Fe₃Al₂(SiO₄)₃, was formerly much used in jewelry and is known by the popular name carbuncle. It occurs in a range of hues from deep red to black, but usually only stones that are both red and transparent are regarded as gems. Some specimens have a violet cast, and some exhibit asterism, showing a four-rayed star in reflected light. Almandine is a widely distributed mineral, found typically in mica schists in India, Australia, South America, and North America.

Uvarovite, Ca₃Cr₂(SiO₄)₃, is an emerald green variety of garnet found primarily in tiny crystals too small for gem use. Comparatively rare, it occurs chiefly in serpentine rocks in the northern Ural Mountains of Russia.

Andradite, Ca₃Fe₂(SiO₄)₃, is a type of garnet that varies widely in composition and color. It occurs in metamorphic rocks in Italy. An opaque, black variety, called melanite, is sometimes used for jewelry by people who are in mourning. A transparent yellow variety that resembles topaz is sometimes misleadingly called topazolite.