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| Postage stamps dedicated to Tanzanite! |
Tanzanite under a microscope!
image location - microscopy.fsu.edu |
Natural tanzanite crystals are dusty brown when found, but after they
are heat-treated at 700 degrees they turn into velvety-blue gems. Being
trichroic, tanzanite will display different colors when viewed at
different angles. Blue, purple or a rusty brown shade will be seen
depending on the orientation of the stone. The blue shade, which brings
the highest price, is displayed across the width of the crystal,
forcing the gem cutter to choose between a small, more valuable stone,
or a larger, less expensive one. If the gem cutter chooses size,
beautiful lavender stones will be the result. Occasionally green or
pink tanzanites are found and sold without going through the heat
treatment process. Faceted tanzanite is generally free of inclusions.
Inclusions in the rough material are cut out as much as possible to
prevent fissures forming during the heating process.
Most tanzanite deposits are found in the Merelani Hills and surrounding areas, with a
few, almost insignificant finds, in nearby Kenya. There are no major
deposits known, and almost all mining is done on a small scale under
primitive conditions. In the mid-eighties the Tanzanian government, in
need of cash, flooded the markets, and caused the prices to fall
dramatically. With tighter restrictions by the state controlled gem
trade, prices rose steadily until 1998 when floods killed several
miners and closed both mines and roads causing prices to skyrocket.
| Composition | Ca2Al3(SiO4)3(OH) | Class | sorosilicate |
| Specific gravity | 3.3 | Refractive Index | 1.68 to 1.72 |
| Birefringence | good (.010) | Pleochroism | strong |
| Cleavage | good on the s axis | Fracture | conchoidal to uneven |
| Luster | vitreous | Transparency | transparent to translucent |
| Streak | white | Fluorescence | none |
| Crystal system | orthorhombic | Twinning | |
| Hardness | 6.5 to 7.0 | Dispersion | .030 |
| Color | Tanzanite is usually blue, lilac blue, or deep violet blue. Occasionally green, yellow, pink, or brown stones reach the market. |
| Crystal habit | Tanzanite may be massive or granular.It often includes long, somewhat prismatic or tabular crystals with a typically dominant pinacoid that the crystal is often flattened against. The terminations are usually poorly developed. |
| Other |
Tanzanite was
discovered in 1968 in the Merelani Hills of Tanzania, in the shadow of
Mt. Kilamanjaro. Throughout the 1970s, this gem was exclusively marketed
by discoverer of the gem, Tiffany & Co. Today tanzanite is mined in
approximately 100 mines in four zones by just over 150 miners. Small
dealers, licensed by the Tanzanian government supply almost all of the
material produced and sold legally. Although there is almost always a
black market where gems are involved, it is kept to a minimum due to
the limited availability of the gem and strict control by the government.
Legends tell of a
lightning strike near the Merelani hills that set surrounding grasslands
on fire. When the Masai herders returned to the area with their
livestock, beautiful blue stones were found strewn about on the ground.
A Portuguese geologist, named DeSouza, traveling to the area, realized
the stones were zoisite and attempted to replicate the heating process
and in doing so produced the tanzanite we are now familiar with.
