Tanzanite is the blue/purple variety of the mineral zoisite belonging to the epidote group.
It was discovered in the Mererani Hills of Manyara Region in Northern Tanzania in 1967,
near the city of Arusha and Mount Kilimanjaro. Tanzanite is used as a gemstone, and naturally-formed
tanzanite is extremely rare, still found only in the Mererani Hills.
Tanzanite is noted for its remarkably strong trichroism, appearing alternately sapphire
blue, violet and burgundy depending on crystal orientation. 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 is usually a reddish brown in its rough state, requiring artificial
heat treatment to bring out the blue violet of the stone.
The mineral was named by Tiffany & Co. after Tanzania, the country in which it was discovered.
In 2002, the American Gem Trade Association chose Tanzanite as a December birthstone,
the first change to their birthstone list since 1912.
Commercial history
Manuel de Souza, a tailor and part-time gold prospector living in Arusha, 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 but quickly realized that it was
not, so he took to calling it "dumortierite", a blue non-gem mineral. Shortly thereafter,
the stones were shown 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. Scientifically called "blue zoisite", the
gemstone was renamed as tanzanite by Tiffany & Co., who wanted to capitalize on the rarity
and single location of the gem, but who thought that "blue zoisite" wouldn't sell well. Tiffanys
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. Recent Tanzanite Mining Developments
In 1990 the Tanzanian government split the Tanzanite mines into four sections: Blocks
A, B, C and D. Blocks A and C were awarded to large operators, while Blocks B and D were
reserved for the local miners. In 2005 the government renewed the lease of Block C mine
to Tanzanite One, who paid 40 million dollars for their lease and mining licence.
In June 2003, the Tanzanian government introduced legislation banning the export of unprocessed
tanzanite to India. 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.
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. Production in 2011 amounted to 2.4 million
carats, earning them $24 million.
The world's largest rough tanzanite was a 16,839 carat stone mined by TanzaniteOne in
2005. 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, 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 violet. 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.
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. 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 treatment Tanzanite forms as a brownish crystal and
is trichroic - which means it shows three colours - brown, blue and violet concurrently.
Heating - either occurring underground naturally by metamorphic occurrences or above ground
by man in a furnace removes the brown or burgundy color component to produce a stronger purplish-blue
color and makes the stone "dichroic" which means it only reflects blue and violet.
When heat treated in a furnace, the temperature is usually between 370-390 degrees Celsius
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 were gem-quality
blue without the need for heat treatment - probably the result of a wild fire in the area which
heated the stones underground. This gave rise to the idea that "D block" stone were more
desirable than tanzanite found in other areas 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 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.
See also Manyara Region
Arusha Region References
External links Time Article of 2007 about the popularity
of tanzanite. Article in the early stages of tanzanite
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
A picture of a purported 737.81 carat faceted tanzanite