GO 340 Gemstones & Gemology
Thulite. Photo date 4/02; © S.W. Aber.
|Tanzanite is a member of the zoisite group, as is thulite, the national gemstone of Norway. Thulite is an opaque, pink gemstone usually cut en cabochon, and is named after Thule, an old name for Norway. The gem quality transparent zoisite, tanzanite, varies from a deep "sapphire" blue to an "amethyst" purple. It is now one of the birthstones accepted for December birthdays and is a calcium aluminum silicate, Ca2Al3(O/OH/SiO4/Si2O7) (Schumann, 1997, p. 160). See The Image for another picture and general information.|
|Tanzanite has a hardness of 6.5-7 and specific gravity of 3.35. It has both cleavage and an uneven fracture, and has an orthorhombic crystal system, which is reflected in the bladed crystal habit. There is no fluorescence, but it does have very strong pleochroism. Tanzanite is trichroic, purple, blue, and bronze colors. Tanzanite is doubly refractive, with refractive indices of 1.691 and 1.70.|
|Although zoisite was first found in Austria in 1805, tanzanite was not found until 1967. Tanzanite is named after the East African nation of Tanzania, found in a hilly area called Merelani, close to Mount Kilimanjaro, and the only locality this gem variety has ever been found (CW Jewelers, 1999 and Jewelery Central, 1999-2011).|
It occurs as the vein or fissure filling mineral in metamorphic gneiss. The intense blue-violet color is caused by vanadium. Both crystal orientation and lighting conditions can reveal a dominant color and color is of prime consideration in the value of tanzanite. The blue or violet-blue are the most valued. These same colors can be found in amethyst, iolite, sapphire, spinel, synthetic corundum, and glass imitations.
Image left is heat-treated tanzanite taken from Gem Hut.
Tanzanite does not have a history or lore because it was only recently discovered. Natural, rough tanzanite is usually a bronze or yellow color and has been known for some time. The new discovery was that heat removed the bronze color and brought out the blue-violet color. Masai herders, who drive cattle in the Merelani hills, came upon blue zoisite crystals after lightning had sparked a fire that swept through the area. This "blue zoisite" became "tanzanite" when it was introduced on the market in 1969, by Tiffany & Company (CW Jewelers, 1999).
Image right is a natural tanzanite crystal taken from Mineral Galleries.
Tanzanite crystal from
There is still much debate about tanzanite sales and tanzanite mining having connections to funding terrorism. While reporters for several large news agencies who first made this connection were branded by some as fabricating stories; the reporters responded that they were revealing the truth about one of many African gemstone commodities sold to raise funds for terrorism. Is this an example of opinionated journalism and misrepresenting quotes in reporting the words from prominent gem leaders as some claim or exemplary reporting and exposing the terrorist links to gem miners, retailers, dealers, and organizations? For more information visit:
|Various lighting conditions have noticable effects on the color of tanzanite. According to (CW Jewelers, 1999), morning sunlight adds red, orange, or yellow, making the gem look more purple; overcast natural light adds blue and gray; incandescent light adds red which strengthens purple colors and turn blue to a violet color; fluorescent light strengthens the blue; halogen lighting adds sparkle and strengthens the purple.|
Tanzanite is a highly valued gemstone and is priced just below emerald, ruby, and sapphire. Tanzanite can be synthesized and it is imitated by treated beryl, glass, and assembled stones. Doublets and triplets are made with a tanzanite crown and colorless synthetic spinel pavilion or two colorless synthetic spinels with a tanzanite colored cement holding the crown and pavilion together.
Image right is the Petersen Tanzanite Brooch. This brooch is located in National Gem and Mineral Collection, Smithsonian National Museum of Natural History, Washington, DC. It has two stunning 15 carat tanzanites, surrounded by numerous diamonds with a total carat weight of 24. The brooch was created by Harry Winston, Inc. in 1991. Image courtesy of Margaret M. Martin; photo date 1/2010.
To learn more about tanzanite visit:
Introductory image taken from Jewelry Central.
Return to the Syllabus or choose another gemstone below.
This page originates from the Earth Science department for the use and benefit of students enrolled at Emporia State University. For more information contact the course instructor, S. W. Aber, e-mail: email@example.com Thanks for visiting! Webpage created: November 15, 2000; last update: September 29, 2011.Copyright 1999-2011 Susan Ward Aber. All rights reserved.