Tanzanite

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Tanzanite
Chemical composition Calcium aluminum silicate
Crystal system Orthorhombic
Habit Prismatic
Cleavage Perfect, prismatic
Fracture Uneven
Hardness 6.5
Optic nature Biaxial +
Refractive index 1.685-1.707
Birefringence 0.008
Dispersion Low, 0.012
Specific gravity 3.35
Lustre Vitreous
Pleochroism Strong trichroism (dichroism in heated)
Faceted D Block Tanzanite
Photo courtesty of Lembeck Gems

Tanzanite, a variety of zoisite, was discovered in the Merelani district of Tanzania near Mount Kilimanjaro in 1967. So far, all tanzanite discovery has been limited to a six kilometer strip in this area, which was divided into four blocks, labeled A, B, C and D. Block C is the only site still commercially producing rough tanzanite at this time.

Almost all purple and blue varieties of the stone are the result of heat treatment. The stone is a yellowish-brown "bug juice" color when it is mined, but when heated to between 426.7-482.2 degrees Celsius, it becomes that gorgeous violet blue that is used in jewelry. The stone has never been found in another locality, so it is indeed a depleting resource. It is somewhat soft (6-7 on Mohs scale of hardness) and has a direction of cleavage along which it can be easily broken. It is desirable because of its elegant, opulent color.

Diagnostics

Tanzanite can be confused with sapphire. Interference figure, refractive index and specific gravity can easily seperate tanzanite from sapphire.

Color

Violet blue to blue to bluish purple. The color is due to vanadium.
Someone once described the color of tanzanite as "Tanzanite looks like sapphire wishes it could look" [Wise, 2003].

Diaphaneity

Transparent.

Magnification

Tanzanite is a type I stone on the GIA clarity scale and is usually not included.

Refractometer

nα = 1.685 - 1.696, nβ = 1.693 -1.702 and nγ = 1.700 -1.707 with a birefringence of 0.008 - 0.013.
The optic nature is biaxial +.

Other stones falling in this range are:

Specific gravity

3.25 - 3.60 (mean 3.35).
Tanzanite will sink slowly in methylene iodide (sg = 3.33).

Spectroscope

Spectrum of blue, transparent tanzanite.

Untreated tanzanite will show weak bands in the yellow-orange, red, green and blue. Heat treated tanzanite may show even weaker bands or non atall (especially in lighter colors).

Pleochroism

Natural tanzanite does occur and it exhibits strong trichroism. The typical colors are violet to purplish red, blue and green (the green portion may also be yellow).
Most tanzanite on the market is heated brownish zoisite and shows two distinct colors under the dichroscope: violet to purplish red and blue. There will be two shades of blue, so the stone is still considered as trichroic, but the yellow or green is not observed in these heated stones. For that reason it is often referred to as being dichroic.

Durability

Tanzanite is relative soft on the Moh's scale (6½) so care should be taken not to scratch it. Due to its perfect prismatic cleavage it can easily break when hit with a hard and/or sharp object.

Phenomena

Cat's eye stones are known to exist

Cat's Eye Tanzanite
Photo courtesty of The Gem Trader

Treatments

Usually heat treated to remove the yellow-green to brown components.

Synthetics

None known (although there are rumors of a synthetic tanzanite being created in Russia).

Imitations

Most stones that are used to simulate tanzanite can easily be seperated by optical properties and/or the use of a Hanneman Tanzanite Filter.


Imitation Key seperation
Synthetic forsterite Double refraction, Tanzanite filter, fluorescence
U.M.Tanzanic (lead glass) Polariscope, RI ~ 1.6
Coranite™ (synthetic sapphire) Refractive index, polariscope
Tanavyte™ (YAG) Polariscope, RI ~ 1.8
Tanzation (synthetic spinel/cobalt glass triplets) Polariscope, RI = 1.726
Cubic zirconia Polariscope, RI = 2.1-2.2, SG = 5.6-5.9

Sources

  • Gem Identification Made Easy, Third Edition (2003) - Antoinette L . Matlins, Antonio C. Bonanno ISBN 0943763347
  • Gemmology, Third Edition (2005) - Peter Read ISBN 0750664495
  • Robert Webster, 1990 (4th ed.) - Gems, Their Sources, Descriptions and Identification ISBN 0750658568 (6th ed.)
  • A Students' Guide to Spectroscopy (2003) - Colin Winter FGA, DGA ISBN 0954485300
  • Gem Reference Guide (1995) - GIA ISBN 0873110196
  • Secrets of the Gem Trade, The Connoisseur's Guide to Precious Gemstones (2003) - Richard W. Wise ISBN 0972822399
  • Insider Gemologist: What Are the Identifying Characteristics of Tanzanite, and How Is It Distinguished from Imitations? (2004) - GIA