Garnet
January's Birthstone
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Garnet Information
... Garnet comes from the Latin word, “granatus” which means grain. That is because many garnet deposits are small grains of red crystals in or on their host rock.
... Garnets have several species, as well as several varieties. They are listed here, with more detailed information after the general descriptions.
Garnet Species
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Pyrope Garnet
CHEMISTRY Mg3Al2Si3O12
CRYSTALLOGRAPHY Isometric
REFRACTIVE INDEX 1.73 - 1.76
HARDNESS 7 - 7.5
SPECIFIC GRAVITY 3.65- 3.87
DISPERSION 0.022
CLEAVAGE None
HEAT SENSITIVE Some.
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Pyrope
... Pyrope comes from a Greek word meaning “fire like.” The common dark red garnets are a mixture of pyrope and almandine.
... One popular garnet is chrome pyrope, whose color rivals ruby. These are found in Arizona, where ants bring them to the surface. Hence, they are dubbed, “ant hill garnets.” While their color is superb, they are very dark in tone. Gems are rarely faceted in sizes over one carat because of this.
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Almandine Garnet
CHEMISTRY Fe3Al2Si3O12
CRYSTALLOGRAPHY Isometric
REFRACTIVE INDEX 1.75 - 1.83
HARDNESS 7 - 7.5
SPECIFIC GRAVITY 3.95- 4.30
DISPERSION 0.024
CLEAVAGE None
HEAT SENSITIVE Some.
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Almandine
... One of the classic sources of this garnet is Alabanda, in Asia Minor. Its common name is a modification of the source. The Roman historian Pliny wrote of them.
... Our common, dark red garnets are a blend of almandine and pyrope. Throughout history, this has been one of the most popular gems. They are found world wide and in great abundance. Hence, the value is low.
... Very large crystals exist, but because of their dark tone, only small to medium sized gems are faceted. These are cut very shallow, to let light pass through.
... Almandine garnets from Idaho and India sometimes have asbestos fiber inclusions. These will produce star stones when properly cut. They are highly prized by collectors, because of their rarity. They are also one of the most difficult gems to cut.
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Spessartite Garnet
CHEMISTRY Mn3Al2Si3O12
CRYSTALLOGRAPHY Isometric
REFRACTIVE INDEX 1.79 - 1.83
HARDNESS 7 - 7.5
SPECIFIC GRAVITY 3.80- 4.25
DISPERSION 0.027
CLEAVAGE None
HEAT SENSITIVE Some.
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Spessartite
... This garnet is named after Spessart, Bavaria.
... Spessartite is somewhat rare. As with the other garnets, it always occurs in a blend with other species. Gems with the highest spessartite content are a light orange. Those with an almandine content are reddish, to red brown in hue.
... The most valuable spessarties are a bright, orangish red. These come from Ramona, California, and Amelia, Virginia...
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Andradite Garnet
CHEMISTRY Ca3Fe2Si3O12
CRYSTALLOGRAPHY Isometric
REFRACTIVE INDEX 1.86 - 1.95
HARDNESS 6.5 - 7
SPECIFIC GRAVITY 3.70- 4.10
DISPERSION 0.057
CLEAVAGE None
HEAT SENSITIVE Some.
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Andradite
... Andradite is named after the Portuguese mineralogist, d’Andrade.
... This is one of the rarest and most sought after garnets. There are no major sources of andradite and the supply is limited to small deposits.
... Its dispersion is much higher than any other garnet and even much higher than diamond. The dispersion is usually masked by dark body colors but small, light colored gems are dazzling!
... The variety dematoid is colored green by chromium. This gem is always in high demand.
... Andradites are known for their distinctive, horsetail inclusions. (See “Identifying Inclusions” in our Reference Library.) They are both an aid to the gemologist and a delight to collectors.
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Grossular Garnet
CHEMISTRY Ca3Al2Si3O12
CRYSTALLOGRAPHY Isometric
REFRACTIVE INDEX 1.72 - 1.80
HARDNESS 6.5 - 7.5
SPECIFIC GRAVITY 3.40- 3.70
DISPERSION 0.028
CLEAVAGE None
HEAT SENSITIVE Some.
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Grossularite
... The botanical name for gooseberry is grossularia, from which this garnet receives its name.
... Unlike the other garnets, grossulars are rarely red or dark. They come in every color except blue and are sometimes colorless. The tone is often light to medium. They make brilliant gems with vibrant colors...
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Hydrogrossular Garnet
CHEMISTRY Ca3Al2(SiO4)3-x(OH)4
CRYSTALLOGRAPHY Isometric
REFRACTIVE INDEX 1.72 - 1.80
HARDNESS 6.5 - 7.5
SPECIFIC GRAVITY 3.40- 3.70
DISPERSION 0.028
CLEAVAGE None
HEAT SENSITIVE Some.
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Hydrogrossular
... Hydrogrossular differs from the other garnets in that it is never transparent. It ranges from translucent to opaque. The most common color is a bluish green, but they are also found in pink, white, and gray.
... Because of its coloring and translucency, hydrogrossular is often used as a jade substitute. Large pieces are available, which lend themselves to carving...
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Uvarovite Garnet
CHEMISTRY Ca3Cr2Si3O12
CRYSTALLOGRAPHY Isometric
REFRACTIVE INDEX 1.74 - 1.87
HARDNESS 6.5 - 7.5
SPECIFIC GRAVITY 3.40- 3.80
DISPERSION unknown
CLEAVAGE None
HEAT SENSITIVE Some.
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Uvarovite
... Uvarovite is named after Count S. S. Uvarov, (1765-1855), president of the St. Petersburg Academy and mineral collector.
... This is the rarest of the garnet family. Colored by chromium, it is always a dark, rich green. The crystals are small and most people have only seen examples of druzy on matrix.
... The crystals are usually opaque. Only small corners of larger crystals have the transparency for faceting. Anything over one carat is exceptionally rare for a faceted uvarovite. Collectors are grateful to have a faceted uvarovite of any size. They are so rare; there simply are not enough to go around.
... See Natural Garnet gemstones of all varieties.
Garnet Varieties
Rhodolite
... Some say the name rhodolite comes from the Greek word, rhodon, meaning rose. Other scholars compare the name to rhododendron. In either case, the name is comparing the color to a flower.
Malaia
... Malaia is a Bantu word that means out of the family, or out of the tribe. It is also used to mean prostitute or deceiver. It came into usage for a number of garnets that did not fit into any of the standard categories.
Dematoid
... Dematoid comes from the French, “demant,” meaning diamond. The reason is obvious, with its high brilliance and dispersion. Dematoid garnets are a green variety of andradite. They are known for their golden, “horsetail” inclusions.
Tsavorite
... Tsavorite is named after its only source, the Tsavo Valley in Kenya. It is the chromium colored, green variety of grossular.
... These popular gems demand high value in today’s market. While faceted stones approaching 20 carats are known, their deep coloring usually keeps their size below three carats.
Hessonite
... Hessonite is from a Greek word meaning inferior. This refers to it having less hardness than other garnets.
... Hessonites are an orangish variety of grossular garnet. Sometimes their coloring leans towards the pink. Asbestos, Quebec is one of the most common sources. The miners find pinkish orange crystals among the asbestos. Africa is also a major source for hessonite.
Color Change Garnets
... Any gem that changes color is a rare find and a treat for collectors. Garnets exhibit the widest variety of color changes in the gem world, with almost every hue exhibited.
... It is commonly said that garnets come in every color of the rainbow except blue. This is still true in natural light, but there are recent discoveries of garnets that turn blue in artificial light.
... Color change garnets are mostly pyrope and spessartite in composition. Except for the color change, they are identical in properties to the Malaia variety. Their primary source is Africa.
... Idaho garnets, which are primarily almandine/pyrope mixtures, occasionally show a strong color shift from red to purplish red.
Proteus
... In Greek mythology, Proteus was a sea god, capable of changing his shape. It has become a noun for one who easily changes their appearance or principles.
... Proteus are the only treated garnets. All the others resist change, but a few almandine/pyropes from the US will change into Proteus. The treatment brings a thin layer of metals to the surface.
... This causes it to have a dual appearance. In reflected light, they have a dark gray, metallic luster, much like hematite. In transmitted light, the dark red of the garnet shows through.
... See Natural Garnet gemstones of all varieties.
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