Tanzanite
| Tanzanite | |
|---|---|
.jpg) Tanzanite rough stone and cut stone | |
| General | |
| Category | Sorosilicate |
| Formula (repeating unit) | (Ca2Al3(SiO4)(Si2O7)O(OH)) |
| Strunz classification | 09.BG.10 |
| Crystal system | Orthorhombic |
| Identification | |
| Color | Purple to blue |
| Crystal habit | Crystals flattened in an acicular manner, may be fibrously curved |
| 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. |
Tanzanite is the blue/purple variety of the mineral zoisite (a calcium aluminium hydroxy silicate) which was discovered in the Mererani Hills of Northern Tanzania in 1967, near the city of Arusha and Mount Kilimanjaro. 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.[1] Tanzanite can also appear differently when viewed under alternate lighting conditions. The blues appear more evident when subjected to fluorescent light and the violet hues can be seen readily when viewed under incandescent illumination. 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.[2] Tanzanite is a rare gem.[3][4] It is found mostly in the foothills of Mount Kilimanjaro. The mineral is named after Tanzania, the country in which it was discovered.
Commercial history
Manuel de 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 possibilities, 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 University, 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.[5][6]
Officially called "blue zoisite" it was marketed as tanzanite by Tiffany & Co., who wanted to capitalize on the rarity of the gem, then only found in Tanzania, but who thought that "blue zoisite" (which might be pronounced like "blue suicide") wouldn't sell well.[7] From 1967 to 1972, an estimated two million carats of tanzanite were mined in Tanzania before the mines were nationalized by the Tanzanian government.
World's largest tanzanite
The world's largest faceted tanzanite is 737.81 carats.[8] One of the most famous large tanzanites (242 carats) 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 usually set in necklaces and earrings.[9] 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.[10]
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]
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 five 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-ever tanzanite crystal was found in the C-Block mine. The crystal weighs 16,839 carats (3.4 kg) and measures 8.7 in x 3.1 in x 2.8 in (22 cm × 8 cm × 7 cm).
The mining of tanzanite nets the Tanzanian government approximately US$20 million annually. The finished gems are sold mostly on the US market: sales total approximately US$500 million annually.[citation needed]
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.[11]
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 20/20 vision).[12] 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.
Heat treatment
Excepting stones 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-violet to violetish-blue. Since heat treatment is universal, it has no effect on price, and finished gems are assumed to be heat-treated. Tanzanite is sometimes found in other colors, such as green, although technically it would be called 'green zoisite' rather than tanzanite.[13] 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.[14] Tanzanite is heat treated in a furnace with a temperature between 550 and 700 degrees Celsius. It should not have any cracks or bubbles in it, as it could shatter or the cracks/ bubble will increase in size.
References
- ^ E. Skalwold. "Pleochroism: trichroism and dichroism in gems". Nordskip.com. Retrieved 2011-08-29.
- ^ "YourGemologist / International School of Gemology Study of Heat Treatment". Yourgemologist.com. Retrieved 2011-08-29.
- ^ http://web.archive.org/web/20090131195920/http://nationaljewelernetwork.com/njn/content_display/colored-stones/color-market-reports/e3i35bdd2d07c219452caefa3f76a183a6d?imw=Y
- ^ http://bworldonline.com/Weekender020609/main.php?id=focus1[dead link]
- ^ "The Mineralogical Record - Merelani, Tanzania".
- ^ "Tanzanite: Its discovery and early days - ICA's InColor Magazine. Summer 2007".
{{cite web}}: Missing or empty|url=(help) - ^ "Tanzanite". Gemstone.org. Archived from the original on 2008-09-12. Retrieved 2011-08-29.
- ^ http://www.palagems.com/Images/mineral_news/munich09_tanz.jpg
- ^ "2005 Gem News Archive". Palagems.com. Retrieved 2011-08-29.
- ^ "Royal Ontario Museum | Exhibitions & Galleries | Past Exhibitions | Light & Stone: Gems from the Collection of Michael Scott". Rom.on.ca. Retrieved 2011-08-29.
- ^ 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
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.
- Gem Slaves - a short film from 2006 on tanzanite's child miners