Tanzanite
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| Tanzanite | |
|---|---|
 Tanzanite gemstone, featuring an oval mixed cut |
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| General | |
| Category | Mineral Variety |
| Chemical formula | (Ca2Al3(SiO4)(Si2O7)O(OH)) |
| Identification | |
| Color | Purple to Blue |
| Crystal habit | Crystals flattened in an acicular manner, may be fibrously curved |
| Crystal system | Orthorhombic |
| Cleavage | Perfect {010} imperfect {100} |
| Fracture | Uneven to conchoidal |
| Mohs scale hardness | 6.5 |
| Luster | Vitreous, pearly on cleavage surfaces |
| Streak | White or colorless |
| Specific gravity | 3.10-3.38 |
| Optical properties | biaxial positive |
| Refractive index | 1.69-1.70 |
| Birefringence | 0.006-0.018 |
| Pleochroism | Present, dichroism or trichroism depending on color. |
| Other characteristics | only found in Tanzania |
Tanzanite is the blue/purple variety of the mineral zoisite which was discovered in the Meralani (Merelani) Hills of Northern Tanzania in 1967, near the city of Arusha. It is used as a gemstone. Tanzanite is noted for its remarkably strong trichroism, appearing alternately sapphire blue, violet, and burgundy depending on crystal orientation.
Tanzanite in its rough state is usually a reddish brown color. It requires artificial heat treatment to 600 °C in a gemological oven to bring out the blue violet of the stone.[citation needed] Tanzanite is a rare gem.[1][2] It is found mostly in the foothills of Mount Kilimanjaro.
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[edit] Commercial history
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This section does not cite any references or sources. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. (February 2009) |
Manuel D'Souza, a Goan tailor and part-time gold prospector living in Arusha (Tanzania), found transparent fragments of vivid blue and blue-purple gem crystals on a ridge near Mererani, some 40 km southeast of Arusha. He decided that the mineral was olivine (peridot) but quickly realized that it was not, so he took to calling it "dumortierite", a blue non-gem mineral. Shortly thereafter, D'Souza showed the stones to John Saul, a Nairobi-based consulting geologist and gemstone wholesaler who was then mining aquamarine in the region around Mount Kenya. Saul, with a Ph.D. from M.I.T., who later discovered the famous ruby deposits in the Tsavo area of Kenya, eliminated dumortierite and cordierite as possible I.D.s and sent samples to his father, Hyman Saul, vice president at Saks Fifth Avenue in New York. Hyman Saul brought the samples across the street to the Gemological Institute of America who correctly identified the new gem as a variety of the mineral zoisite. Correct identification was also made by mineralogists at Harvard, the British Museum and Heidelberg University, but the very first person to get the identification right was Ian McCloud, a Tanzanian government geologist based in Dodoma. From 1967 to 1972, an estimated two million carats of Tanzanite were mined in Tanzania. The mines were later nationalized by the Tanzanian government.
[edit] World's largest tanzanite
The world's largest faceted Tanzanite (242 Karats) is the "Queen of Kilimanjaro". It is set in a tiara and accented with 803 brilliant cut tsavorite garnets and 913 brilliant cut diamonds. Because tanzanite is relatively soft, it is not usually set in necklaces and earrings, so the tiara is truly a rarity[3] The tiara is currently on display in the Gallery of Gold and Gems at the Royal Ontario Museum in Toronto, Ontario, Canada. The exhibition is from the private collection of Michael Scott, the first CEO of Apple Computers[4]. Scott's passion for gems began in Florida university, when he was told “You may not wear jewelery, especially rings” while working with computers, because rings could dent the edges of the punch cards used at that time and jam the machine.[5]
[edit] Recent developments
In June 2003, the Tanzanian government introduced legislation banning the export of unprocessed tanzanite to India (like many gemstones, most tanzanite is cut in Jaipur). The ban has been rationalized as an attempt to spur development of local processing facilities, thereby boosting the economy and recouping profits. This ban was phased in over the next two years, until which time only stones over 0.5 grams were affected.[citation needed]
This is a serious situation for the city of Jaipur, as one-third of its annual gem exports are of tanzanite. Some members of the industry fear the ban will set a precedent, leading Tanzania to ban the export of all raw gem material, including the country's production of tsavorite, diamond and ruby.[citation needed]
In April 2005, a company called TanzaniteOne Ltd. publicly announced that they had taken control of the portion of the tanzanite deposit known as "C-Block" (the main deposit is divided into 5 blocks). Prices for rough material on the open market have increased steadily for the last several years as the company has solidified its control of the market. In August 2005 the largest tanzanite crystal was found in the C-Block mine. The crystal weighs 16,839 carats (3.4 kg) and measures 22 cm x 8 cm x 7 cm.
The mining of tanzanite nets the Tanzanian government approximately USD20 million annually. The finished gems later being sold mostly on the US market for sales totaling approximately USD500 million annually.[citation needed]
[edit] Factors affecting value: grading
There is no universally accepted method of grading colored gemstones. TanzaniteOne, a major commercial player in the tanzanite market, through its no-profit subsidiary, The Tanzanite Foundation, has introduced its own color grading system. The new system's color-grading scales divide tanzanite colors into a range of hues, between blue violet and violet blue.
The normal primary and secondary hues in tanzanite are blue and purple not violet. Purple is a modified spectral hue that lies halfway between red and blue. Tanzanite is a trichroic gemstone, meaning that light that enters the stone is divided into three sections each containing a portion of the visible spectrum. After heating, tanzanite becomes dichroic. The dichroic colors are red and blue. The hue range of tanzanite is blue-purple to purple-blue.[6]
Clarity grading in colored gemstones is based on the eye-clean standard, that is, a gem is considered flawless if no inclusions are visible with the unaided eye (assuming 20x20 vision).[7] The Gemological Institute of America classifies tanzanite as a Type I gemstone, meaning it is normally eye-flawless. Gems with eye-visible inclusions will be traded at deep discounts.
[edit] Heat Treatment
Excepting a few stones that were found close to the surface in the early days of the discovery, Tanzanite is universally heat treated to produce a range of hues between bluish-purple to purplish-blue. Since heat treatment is universal, it has no effect on price, and finished gems are assumed to be heat treated. Rarely, tanzanite is found in other colors, green being the rarest, although technically it would be called 'green zoisite' rather than tanzanite.[8] Tanzanite may be subjected to other forms of treatment as well. Recently coated Tanzanites were discovered and tested by the AGTA and AGL laboratories. A thin layer of coatings was applied to improve the color of the Tanzanite.[9]
[edit] Simulants
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This section does not cite any references or sources. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. (February 2009) |
A lab-created simulant of tanzanite is called tanzanique. It closely mimics the color of natural tanzanite however it does not display the same pleochroism. Tanzanite is the mineral zoisite, while tanzanique is forsterite. Several different hues of cubic zirconia have also recently come into general use as tanzanite simulants. A popular hue is a medium-light periwinkle blue/lavender color. Another is a deeper purplish-blue. New hue varieties will likely be developed, or at least, the existing ones will become more widely available, as the supply of tanzanite depletes over time and public awareness and interest in the gem grows.
[edit] References
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Wikimedia Commons has media related to: Tanzanite |
- ^ http://www.nationaljewelernetwork.com/njn/content_display/colored-stones/color-market-reports/e3i35bdd2d07c219452caefa3f76a183a6d?imw=Y
- ^ http://bworldonline.com/Weekender020609/main.php?id=focus1
- ^ http://www.palagems.com/gem_news_2005.htm
- ^ http://www.rom.on.ca/exhibitions/special/lightandstone.php
- ^ http://www.rom.on.ca/media/magazine/fall08.php
- ^ Wise, Richard W., Secrets Of The Gem Trade, The Connoisseur's Guide To Precious Gemstones, p.220
- ^ Wise, Richard W., Secrets Of The Gem Trade, The Connoisseur's Guide To Precious Gemstones, p.35
- ^ GIA On Tanzanite 1999 GIA's short reference list on Tanzanite
- ^ Tanzanite Coating Farlang's/Gem and Diamond Foundation reporting on AGTA and AGL tests
[edit] External links
- Time Article of 2007 about the popularity of tanzanite.
- Article in the early stages of tanzanite (1969)
- Interesting article and documents from the early days: "Something new out of Africa but no one knew what it was".
- The ICA's tanzanite information page.
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