Garnet refers to a group of minerals commonly found in metamorphic rock and in ultramafic igneous rock formations. Garnet is classified as a Nesosilicate in the Silicate mineral group. There are six common varieties of garnet that are identified by their chemical composition and color. They are almandine, andradite, grossularite (tsavorite), pyrope, spessartite, and uvarovite.
Red (pyrope) garnet has a hardness of 7 to 7.5 on the Mohs scale. The toughness of Garnet is Fair to Good. Garnets crystallizes in the isometric or cubic crystal system, and have a crystal habit which is primarily dodecahedral and secondarily hombohedral or trapezohedral. Pyrope Garnet has a vitreous luster, Andradite and Uvarovite have an adamantine luster. Pyrope garnet gemstones are often totally free of inclusions. Garnet has a refractive index of 2.168.
The name "Garnet" (Grenat French, Granat German, Granate Spanish) comes from the Latin word for pomegranate, malum granatum, due to the resemblance of some varieties of garnets to red pomegranate seeds. Red is the most common color of Garnet but they can be found in a variety of colors. Garnet colors (below) include: Golden Andradite, Chrome Pyrope (red), Mozambique, Rhodolite (magenta), Spessartite (yellow/orange) and Tsavorite (green). There is also an extremely rare form of color-change (pleochroism) garnet (shifting from blue to red) found in Madagascar. Garnet has been used by man since the late Bronze Age, and its use as gemstone dates back to ancient Egypt.
Garnet Photo (left) - Dan Dennis
Rhodolite
Rhodolite (aka pyrope-almandine garnet) is a variety of garnet with a "raspberry red" purplish-pink hue (above, right) that is a combination of almandite and pyrope garnet, containing trace amounts of aluminum (Al) and magnesium (Mg) as allochromatic coloring agents. Primary sources for Rhodolite are found in Kenya, Mozambique, Sri Lanka (Ceylon), the United States, and Zimbabwe. The name Rhodolite comes from the Greek "rhodon lithos" or "rose stone."
Tsavorite
Tsavorite is a very rare, recent mineral find, first discovered near Komolo, in the Merelani Hills, 50 km south-east of Arusha in Tanzania. British geologist Campbell Bridges found the first piece of tsavorite, in 1967. Tsavorite (aka tsavolite) is a green grossularite variety of garnet with an intense emerald-green, grass-green, or bluish-green hue (above, left) that is a combination of calcium and aluminum, containing trace amounts of chromium (Cr) and vanadium (V) as allochromatic coloring agents.
The primary source for Tsavorite is located in bush-country of south-eastern Kenya and north-eastern Tanzania. The name Tsavorite was coined by Tiffany & Company's president, Henry Platt in 1974; named for Tsavorite's discovery in the Tsavo National Park, Kenya. Tsavorite is reminiscent of emerald but with higher brilliance, fire, durability, and a total lack of inclusions.
Mandarin Garnet
Mandarin Garnet is another rare, and very recent mineral discovery, having been first identified in 1991. Mandarin Garnet (aka Kunene Spessartines) is a very rare variety of garnet with a distinct "pure orange" saturated hue (above, right) that is a manganese (Mn) aluminium (Al) variety of garnet. Spessartite's orange color is ideochromatic.
The primary source for Mandarin Garnet was found in alluvial secondary deposits along the Kunene River between Namibia and Angola, in south-west Africa, but this location appears to be mined out. Other sources for Mandarin Garnet or orange spessartines have been discovered in south-western Nigeria in 1994 and more recently in Madagascar and Sri Lanka.
Spessartite specimens from Nigeria have a more reddish-orange hue and stones from Madagascar and Sri Lanka are more of an orange-red with brownish overtones that are considered inferior to a true 'orange' Mandarin garnets from Namibia. The name Mandarin Garnet was coined for Spessartite's deep Mandarin-orange color, and the name "Spessartite" comes from the Spessart mountains in northwestern Bavaria.
Rainbow Andradite
A rare variety of calcium iron garnet called Rainbow Andradite (aka Rainbow Garnet) exhibits unusual iridescence, reflecting and refracting the full spectrum of light in a 'rainbow' play-of-color. The crystals are an unspectacular orange-brown to greenish-gray, with a lamellar structure causing " thin-film interference" that is responsible for the iridescent quality. Rainbow Andradite was first discovered in the 1930s, at Nevada's Andradite mine. The only current sources for Andradite are the village of Tenkawa in Nara Japan, about 30 km south of Kyoto, and in Sonora, Mexico.
Chemical composition:
Red Garnet (Pyrope): Mg3Al2[SiO4]3
Rhodolite (Almandine): Fe3Al2(SiO4)3
Tsavorite (Grossular): Ca3Al2(SiO4)3
Mandarin Garnet (Spessartite): Mn3Al2(SiO4)3
Garnets are one of the few gemstones that are rarely treated or enhanced. For more information on gemstone enhancement go to: Gemstone Enhancement & Treatment Techniques.
Referance Credits & Suggestions for Further Study on Gemstones
Judith Crowe, The Jeweler's Directory of Gemstones 
. DK Publishing.
Richard W. Wise, Secrets of the Gem Trade: Connoisseur's Guide to Precious Gemstones . Brunswick House
Walter Schumann, Gemstones of the World . NAG Press; 2Rev Ed edition
Renee Newman, Gemstone Buying Guide . International Jewelry Publications; 2nd edition
Antoinette L . Matlins, Antonio C. Bonanno, Gem Identification Made Easy . Gemstone Press; 3rd edition
Paul R. Shaffer, Herbert S. Zim, Raymond Perlman, Rocks, Gems and Minerals . Martin's Press
Tucson Gem Show, Colored Gemstone News . www.tucsonshowguide.com
Kyoto University, Rainbow Garnet from Tenkawa Village in Nara. www.gaaj-zenhokyo.co.jp
