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Roger Weller, geology instructor                     

Michael Schweska
Physical Geology
Spring 2005

                                  December Birthstone-Tanzanite

Tanzanite In October 2002, Tanzanite was added to the official birthstone list by the American Gem Trade Association as an additional birthstone for December. Tanzanite was first discovered in 1967 in Tanzania, East Africa. As one of the newest and most exotically colored gemstones, Tanzanite is part of the Zoisite mineral species. Tanzanite is a brittle stone and although it can be worn daily, care should be taken to protect it from knocks, pressure and extreme temperature changes. Do not use a home ultrasonic to clean jewelry with Tanzanite. Tanzanite is very rarely a pure blue and usually exhibits rich purple overtones. Tanzanite is 6.5 - 7 on the Mohs scale of hardness. The only known source of Tanzanite is a five square mile hilltop at Merelani, ten miles south of the Kilimanjaro International Airport in Tanzania. Click here for a picture in natural form. Click here for more faceted and natural specimens.


When first mined, most stones are a muted green color, often a blue color and it can also be brown. Tanzanite is routinely heat treated to permanently draw out its exotic bluish-purple color. It has better fire than the Tourmaline elbaite or Peridot and an adequate hardness. Its only one direction of cleavage is somewhat of a problem because it is oriented with the direction of strongest pleochroism. Pleochroism is very pronounced in Tanzanite and is seen as three different color shades in the same stone. In the viewing a tanzanite stone, the colors dark blue, green-yellow and red-purple can be seen, all a result of pleochroism. Lesser stones may have a brownish color due to the mixing of blue, purple and green. This would be a problem in most gemstones because that is the direction the gemcutter would usually select to maximize the color. However, with tanzanite the color is usually strong enough anyway.

Iolite is a blue-violet gemstone variety of the mineral cordierite, has strong pleochroism and can be confused with Tanzanite. However, Iolite is usually less strongly colored, its pleochroic colors vary from blue-violet to yellowish gray to blue and it has less fire. Iolite's unusual color shades makes it an exotic colored gemstone whose popularity is growing day by day. Tanzanite is named after the only location it is found, the country of Tanzania.


The formula for Tanzanite is variety of Zoisite, Ca2Al3(SiO4)3(OH), Calcium Aluminum Silicate Hydroxide. Zoisite, like all epidote minerals, is a structurally complex mineral having both single silicate tetrahedrons, SiO4, and double silicate tetrahedrons, Si2O7. The formula of Zoisite could be expressed in a such a way so as to reflect this organization; Ca2AlOAl2(SiO4)(Si2O7)(OH). Zoisite has been known for nearly two centuries as as a sometimes ornamental stone of limited distribution. Only in 1967 was the blue gemstone variety found in Tanzania. The variety was a surprise to minerologists and gemologists alike in that it had come from a very ungemstone-like mineral. The blue-lavender color of Tanzanite is unique and sets it apart from the other gemstones. Besides tanzanite, Zoisite has produced other attractive specimens that are of interest to collectors. A pink variety called Thulite is usually massive and used for beads and cabochons. A brilliant green variety is associated with medium grade Rubies and is quite popular as an ornamental stone. The Red Rubies are often distorted and irregularly spread throughout the sea of massive Green Zoisite. It is one of the most colorful of ornamental stones and competes well with the popular pink Tourmaline and lavender Lepidolite of California.

Legends, Myths and Healing Properties:

TanzaniteLegend says that it was Masai cattle herders that first noticed this stone some 30 years ago after a fire caused by lightening burned areas in Tanzania. The herders noticed that brown zoiscite crystals had turned a deep blue-purple due to the heat from the fire. Because this is such a new gemstone, there is little folklore or healing properties surrounding this stone.



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Smithsonian Institute 1998-2005 Bernardine Fine Art Jewelry. 1999-2004 Jewelry Central, JC Store.
2004 Amethyst Galleries, Inc. 1995-2005 Loretta Elaine's Gems for Friends.