Garnet is a group of minerals showing crystals with a habit of dodecahedrons and trapezohedrons. They have the same general formula, orthosilicates, and are to a limited degree isomorphous. Many different chemical elements are included in the several varieties of garnet, including calcium, magnesium, aluminium, ferrous or ferric iron, chromium, manganese and titanium. While garnets show no cleavage, a dodecahedral parting is sometimes noted. Fracture is conchoidal to uneven; some varieties are very tough and are valuable for abrasive purposes. Hardness is 6.5-7.5, specific gravity is 3.1-4.3, lustre is vitreous to resinous, and they can be transparent to opaque. Garnets come in red, yellow, brown, black, green, or colorless. The name "garnet" comes from the latin granatus, a grain.
Six varieties of garnet are generally recognized, based on their chemical composition; grossularite (also called hessonite or cinnamon-stone), pyrope, almandine or carbuncle, spessartite, uvarovite and andradite.
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Pendant in uvarovite, a rare bright-green garnet. The long dimension is 2 cm (0.8 inch). |
Grossularite is a calcium-aluminium garnet with the formula Ca3Al2(SiO4)3, though the calcium may in part be replaced by ferrous iron and the aluminium by ferric iron. The name grossularite is derived from the botanical name for the gooseberry, grossularia, in reference to the green garnet of this composition that is found in Siberia. Other shades include cinnamon brown, red, and yellow. Because of its inferior hardness to zircon, which the yellow crystals resemble, they have also been called hessonite from the Greek meaning inferior. Grossularite is found in crystalline limestone with vesuvianite, diopside, wollastonite and wernerite.
Pyrope, sometimes called Cape ruby, is ruby-red in color and chemically a magnesium aluminium silicate with the formula Mg3Al2(SiO4)3, though the magnesium can be replaced in part by calcium and ferrous iron. The color of pyrope varies from deep red to almost black. Transparent pyropes are used as gemstones. The name pyrope is derived from the Greek word meaning "fire-like." A variety of pyrope from Macon County, North Carolina is of a violet-red shade and has been called rhodolite, from the Greek meaning "a rose." In chemical composition it may be considered as essentially an isomorphous mixture of pyrope and almandite, in the proportion of two molecules pyrope to one molecule almandite.
Almandite, sometimes called almandine, is the modern gem known as carbuncle (though originally almost any red gemstone was known by this name). The term "carbuncle" is derived from the latin meaning "little spark." The name Almandite is a corruption of Alabanda, a region in Asia Minor where these stones were cut in ancient times. Chemically, almandite is an iron-aluminium garnet with the formula Fe3Al2(SiO4)3; the deep red transparent stones are often called precious garnet and are used as gemstones. Almandite occurs in metamorphic rocks like mica schists, associated with minerals such as staurolite, kyanite, andalusite, etc.
Spessartite is manganese aluminum garnet, Mn3Al2(SiO4)3. The name is derived from Spessart in Bavaria. Spessartite of a beautiful orange-yellow is found in Madagascar. Violet-red spessartites are found in rhyolites in Colorado and Maine.
Uvarovite is a calcium chromium silicate with the formula Ca3Cr2(SiO4)3. It is a rather rare garnet, bright green in color, usually found as small crystals associated with chromite in serpentines or sometimes in crystalline limestones or schists.
Andradite is a calcium-iron garnet, Ca3Fe2(SiO4)3, is of variable composition and may be red, yellow, brown, green or black. The recognized subvarieties are topazolite (yellow or green), demantoid (green) and melantite (black). Andradite is found both in deep-seated igneous rocks like syentite as well as serpentines, schists, and crystalline limestone. Demantoid has been called the "emerald of the Urals" from its occurrence there.