Tanzanite

Tanzanite
Tanzanite rough stone and cut stone
General
Category	Sorosilicate
Formula (repeating unit)	(Ca2Al3(SiO4)(Si2O7)O(OH))
Strunz classification	09.BG.10
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.

Tanzanite is the blue/purple variety of the mineral zoisite (a calcium aluminium hydroxy silicate) discovered in the Mererani Hills of Manyara Region in 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 only in the foothills of Mount Kilimanjaro. The mineral was named by Tiffany & Co. after Tanzania, the country in which it was discovered. Because it is relatively soft, tanzanite is most commonly set in necklaces and earrings.^[5]

Contents 1 Commercial history 1.1 World's largest tanzanite 1.1.1 Recent developments 2 Factors affecting value: grading 2.1 Heat treatment 3 Trivia 4 See also 5 References 6 External links

Commercial history

The craft work on tanzanite.

Emmanuel Merishiek Mollel, a Maasai 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.^[6][7]
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.^[8] 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.^[9] 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. The piece is part of 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 a two-year period, 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 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 × 3.1 in × 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]}

A rough sample of tanzanite.

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 non-profit subsidiary, The Tanzanite Foundation,^[11] has introduced its own color-grading system.^[12] 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 purple and blue. The hue range of tanzanite is blue-purple to purple-blue.^[13]
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).^[14] 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

Tanzanite is universally heat treated in a furnace, with a temperature between 550 and 700 degrees Celsius, to produce a range of hues between bluish-violet to violetish-blue. Some stones found close to the surface in the early days of the discovery were gem-quality blue without the need for heat treatment. The stones should not have any cracks or bubbles, as they could shatter or the cracks/ bubble could increase in size.
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.^[15] Tanzanite may be subjected to other forms of treatment as well. Recently, coated Tanzanites were discovered and tested by the AGTA and AGL laboratories.^[16] A thin layer containing cobalt, determined by X-ray fluorescence, had been applied to improve the color. It was noted that "coatings in particular are not considered permanent", and in the United States are required to be disclosed at the point of sale.

Trivia

Tanzanite is the unofficial region gem of Manyara Region, where it was discovered and is still mined.

See also

• Manyara Region
• Arusha Region

References

1. ^ E. Skalwold. "Pleochroism: trichroism and dichroism in gems". Nordskip.com. Retrieved 2011-08-29.
2. ^ "YourGemologist / International School of Gemology Study of Heat Treatment". Yourgemologist.com. Retrieved 2011-08-29.
3. ^ http://web.archive.org/web/20090131195920/http://nationaljewelernetwork.com/njn/content_display/colored-stones/color-market-reports/e3i35bdd2d07c219452caefa3f76a183a6d?imw=Y
4. ^ http://bworldonline.com/Weekender020609/main.php?id=focus1^{[dead link]}
5. ^ "2005 Gem News Archive". Palagems.com. Retrieved 2011-08-29.
6. ^ "The Mineralogical Record - Merelani, Tanzania".
7. ^ "Tanzanite: Its discovery and early days - ICA's InColor Magazine. Summer 2007".
8. ^ "Tanzanite". Gemstone.org. Archived from the original on 2008-09-12. Retrieved 2011-08-29.
9. ^ http://www.palagems.com/Images/mineral_news/munich09_tanz.jpg
10. ^ "Royal Ontario Museum | Exhibitions & Galleries | Past Exhibitions | Light & Stone: Gems from the Collection of Michael Scott". Rom.on.ca. Retrieved 2011-08-29.
11. ^ The Tanzanite Foundation
12. ^ Tanzanite One Introduces Quality Grading, Pricing Changes and a Sight System JCK Online, 2005