Tanzanite
Tanzanite is the blue/violet variety of the mineral zoisite (a calcium aluminium hydroxyl Sorosilicate) belonging to the epidote group. It was discovered by a Tanzanian Jumanne Mhero Ngoma 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 relatively cheap gemstone, where it can substitute for the far more expensive sapphire after undergoing artificial heat treatment to form a deep blue coloration. Naturally formed tanzanite is extremely rare[3] and is endemic only to the Mererani Hills.
Tanzanite is noted for its remarkably strong trichroism, appearing alternately sapphire blue, violet and burgundy depending on crystalorientation. 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.
Manuel d'Souza, a 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 (25 mi)
southeast of Arusha. He
assumed that the mineral was olivine (peridot) but
after soon realizing it wasn't, he concluded it was "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, and thought that "blue
zoisite" (which might be pronounced like "blue suicide")
wouldn't 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.
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 license.
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. In 2010, the government of Tanzania banned the export of
rough stones weighing more than one gram.
Tanzanite One
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 2.4 million carats,
earning them $24 million.
The world's largest rough tanzanite was a 16,839 carat
(3.38 kg, or 7.46 lb) stone mined by Tanzanite One 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 bluish-violet and
violetish-blue.
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 color 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, blue, cyan, green, yellow, brown and red. After
heating, tanzanite becomes dichroic.
The dichroic colors range from violet through bluish-violet and violetish-blue
to 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 (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 treatment
Tanzanite forms as a brownish crystal and is trichroic -
which means it shows three colors - 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. Rarely,
gem-quality tanzanite will heat to a green primary hue, most always accompanied
by a blue or violet secondary hue. These green tanzanite have some meaningful
value in the collector market, but are seldom of interest to commercial buyers.
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 (in an area now called D block) 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.
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