The garnet group includes a group of minerals that have been used since the Bronze Age as gemstones and abrasives. The name “garnet” may come from either the Middle English word gernet meaning ‘dark red’, or the Latin granatus (“grain”), possibly a reference to the Punica granatum (“pomegranate”), a plant with red seeds similar in shape, size, and color to some garnet crystals.
Six common species of garnet are recognized by their chemical composition. They are pyrope, almandine, spessartine, grossular (varieties of which are hessonite or cinnamon-stone and tsavorite), uvarovite and andradite.
Garnets species are found in many colors including red, orange, yellow, green, blue, purple, brown, black, pink and colorless.
Garnet species’ light transmission properties can range from the gemstone-quality transparent specimens to the opaque varieties used for industrial purposes as abrasives.
Almandine, sometimes incorrectly called almandite, 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 “live coal” or burning charcoal. The name Almandine is a corruption of Alabanda, a region in Asia Minor where these stones were cut in ancient times. Chemically, almandine is an iron-aluminum garnet the deep red transparent stones are often called precious garnet and are used as gemstones (being the most common of the gem garnets). Almandine occurs in metamorphic rocks like mica schists, associated with minerals such as staurolite, kyanite, andalusite, and others. Almandine has nicknames of Oriental garnet, almandine ruby, and carbuncle.
Pyrope (from the Greek pyrōpós meaning “fire-eyed”) is red in color and chemically a magnesium aluminium silicate 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.
A variety of pyrope from Macon County, North Carolina is a violet-red shade and has been called rhodolite, from the Greek meaning “a rose.” Pyrope is an indicator mineral for high-pressure rocks.
Spessartine or spessartite is manganese aluminium garnet, Its name is derived from Spessart in Bavaria. It occurs most often in granite pegmatite. Spessartine of an orange-yellow is found in Madagascar.
Grossular is a calcium-aluminium garnet, though the calcium may in part be replaced by ferrous iron and the aluminium by ferric iron. The name grossular 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 (cinnamon stone variety), 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.
One of the most sought after varieties of gem garnet is the fine green grossular garnet from Kenya and Tanzania called tsavorite. This garnet was discovered in the 1960s in the Tsavo area of Kenya, from which the gem takes its name.
Uvarovite is a calcium chromium garnet .This is a rather rare garnet, bright green in color, usually found as small crystals associated with chromite in peridotite, serpentinite, and kimberlites. It is found in crystalline marbles and schists in the Ural mountains of Russia. Uvarovite crystals are generally too small to facet and are left attached to a matrix and incorporated into jewelry as seen below.
The name rhodolite derives from the Greek words rhodo and lithos, respectively meaning rose and stone. Garnet’s history dates back to the Bronze Age. It was first popularized between the reign of Alexander the Great and the conquest of Rome and is still today considered as a classic gem to possess. In the mid 60’s the best quality rhodolites were discovered in northeastern Tanzania ’s Umba River. The deposit is still exploited but only during the dry season when the water level recedes and deeper portions of the Umba Valley can be accessed.
For its beautiful and intense colors (from pink to purplish red), rhodolite is one of the most popular gemstones, and is considered by many as the “Queen of Garnets”.
This article is reproduced by kind permission of Keith Birch www.ksccrystals.com