Tanzanite is the blue and violet variety of the mineral zoisite (a calcium aluminium hydroxyl sorosilicate), caused by small amounts of vanadium. Tanzanite belongs to the epidote mineral group. Tanzanite is only found in Simanjiro District of Manyara Region in Tanzania, in a very small mining area approximately long and wide near the Mererani Hills.
Tanzanite is noted for its remarkably strong trichroism, appearing alternately blue, violet and burgundy depending on crystal orientation. Tanzanite can also appear differently when viewed under different 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. In its rough state tanzanite is coloured a reddish brown to clear, and it requires heat treatment to remove the brownish "veil" and bring out the blue violet of the stone.
The gemstone was given the name "tanzanite" by Tiffany & Co. after Tanzania, the country in which it was discovered. The scientific name of "blue-violet zoisite" was not thought to be sufficiently consumer friendly by Tiffany's marketing department, who introduced it to the market in 1968. In 2002, the American Gem Trade Association chose tanzanite as a December birthstone, the first change to their birthstone list since 1912.
Scientifically called "blue zoisite", the gemstone was renamed as tanzanite by Henry B. Platt, a great-grandson of Louis Comfort Tiffany and a vice president of Tiffany & Co., who wanted to capitalize on the rarity and single location of the gem and thought that "blue zoisite" (zoi′sīt) would not sell well. Tiffany's original campaign advertised that tanzanite could now be found in two places: "in Tanzania and at Tiffany's".
From 1967, an estimated two million carats of tanzanite were mined in Tanzania before the mines were nationalized by the Tanzanian government in 1971.
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 reason for the ban is to 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. In 2010, the government of Tanzania banned the export of rough stones weighing more than one gram.
TanzaniteOne Mining Ltd is owned by Richland Resources, but a 2010 law in Tanzania required them to cede 50% ownership of their mining license to the Tanzanian State Mining Company (Stamico). Production in 2011 amounted to , earning them $24 million.
Following the construction of a perimeter wall around the mines, to improve security and prevent smuggling, production rose from in 2018 to a record in 2019.
On 24 June 2020, artisanal miner Saniniu Laizer unearthed two rough tanzanite stones, weighing 11.14 kg (55,700 carats) in total, which he sold to the Tanzanian government’s Ministry of Mining for TSh 7.74 billion (US $3.35 million), breaking the previous record set in 2005 by a 16,839‑carat stone
Total reserves of tanzanite are estimated at , according to a report published in 2018. Block C, by far the largest site, has been estimated at with a Life of Mine (LOM) expected to last until the 2040s.
The normal primary and secondary hues in tanzanite are blue and violet. Untreated tanzanite is a trichroic gemstone, meaning that light that enters this anisotropic crystal gets refracted on different paths, with different colour absorption on each of the three optical axes. As a result of this phenomenon, a multitude of colors have been observed in various specimens: shades of purple, violet, indigo, blue, cyan, green, yellow, orange, red and brown. After heating, tanzanite becomes Dichroism. The dichroic colours range from violet through bluish-violet to indigo and violetish-blue to blue.
Clarity grading in coloured 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). The Gemological Institute of America classifies tanzanite as a Type I gemstone, meaning it is normally eye-clean. Gems with eye-visible inclusions will be traded at deep discounts.
Heat-treating in a furnace is usually carried out at between for 30 minutes. The stones should not have any cracks or bubbles, as they could shatter or the cracks/ bubble could increase in size during furnace heating.
Some stones found close to the surface in the early days of the discovery (in an area now called "D block") were gem-quality blue without the need for heat treatment, probably the result of a wildfire in the area which heated the stones underground. This gave rise to the idea that "D block" stones were more desirable than tanzanite found in other parts of the small tanzanite mining area.
Since heat treatment is universal, it has no effect on price, and finished gems are assumed to be heat-treated. Gemological Institute of America states that the source of heating is gemologically undetectable, but is assumed because of its prevalence.
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 containing cobalt, determined by X-ray fluorescence, had been applied to improve the colour. 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.
The physical characters can make cutting process difficult due to the problem of selecting the 'perfect' color. The finished colour of the gemstone will vary depending on how the table cut reflects the light.
Synthetic forsterite (, the magnesium-rich Endmember of olivine) has also been sold as tanzanite, and presents a similar appearance. It can be distinguished from tanzanite in three ways:
Lower grades of tanzanite are occasionally enhanced by a vacuum deposition coating of cobalt; the cobalt layer imparts a deeper shade of blue. Similar to other gems' optical coating, the cobalt film does not adhere well to these stones: It tends to rub off over time, resulting in a much less intensely colored stone, although still tanzanite. The practice of cobalt coating is considered deceptive unless well-advertised.
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