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16.7.1: Tanzanite

Last updated

May 6, 2022
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- 16.07: Zoisite
- 16.7.2: Thulite

gemology
Gemology Project

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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)

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 separate 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

n_α = 1.685 - 1.696, n_β = 1.693 -1.702 and n_γ = 1.700 -1.707 with a birefringence of 0.008 - 0.013.
The optic nature is biaxial +.

Other stones falling in this range are:

Diopside (higher birefringence)
Sinhalite (higher birefringence)
Idocrase (higher refractive index and lower birefringence)

Specific gravity

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

Spectroscope

Figure $\PageIndex{1}$ : 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 none at all (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 relatively 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.

Treatments

Usually, Tanzanite is heat treated to remove the yellow-green to brown components. Heating is usually conducted at temperatures between 400 and 600 degrees centigrade. As Tanzanite's color largely depends on the ratio of Vanadium and Titanium within its crystal structure, any changes to this delicate balance have a marked effect on its color. Heating results in a valence exchange reaction or redox reaction which causes an increase in the violet and blue colors and renders the Titanium colorless. The image below shows Tanzanite crystals before and after heating.

Figure $\PageIndex{2}$

Tanzanite pictures

Figure $\PageIndex{3}$ : Rought Tanzanite showing burgundy color, untreated, directly in Mirerani.

Figure $\PageIndex{4}$ : Tanzanite ore is bluish, graphite-containing, often with quartz, pyrite, garnet

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 separated by optical properties and/or the use of a Hanneman Tanzanite Filter.

Table $\PageIndex{1}$

Imitation	Key separation
Synthetic forsterite	Double refraction, Tanzanite filter, fluorescence
U.M.Tanzanic (lead glass)	Polariscope, RI ~ 1.6
Coranite™ (synthetic sapphire)	Refractive index, optic figure using polariscope with coniscope
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 3rd edition (2005) - Peter Read

Gems Their Sources, Descriptions and Identification 4th Edition (1990) - Robert Webster (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, by Richard W. Wise
Insider Gemologist: What Are the Identifying Characteristics of Tanzanite, and How Is It Distinguished from Imitations? (2004) - GIA
The Science Behind Tanzanite Heating (2013) - Antony Zagoritis, GG