| Tanzanite | |
|---|---|
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| General | |
| Category | Sorosilicate: zoisite variety |
| Formula (repeating unit) | (Ca2Al3(SiO4)(Si2O7)O(OH)) + (Cr,Sr) |
| Strunz classification | 09.BG.10 |
| Crystal system | Orthorhombic[1] |
| Space group | Pnma (no. 62) |
| Identification | |
| Color | Royal blue, indigo, violet/purple |
| Crystal habit | Prismatic crystals with striations; massive to columnar[1][2] |
| Twinning | penetration twins |
| Cleavage | Perfect {010}, imperfect {100}[1] |
| Fracture | Uneven to conchoidal[1] |
| Mohs scale hardness | 6.5 |
| Luster | Vitreous, pearly on cleavage surfaces |
| Streak | White or colorless |
| Specific gravity | 3.10–3.38 |
| Optical properties | biaxial positive |
| Refractive index | 1.69–1.70 |
| Birefringence | 0.006–0.018 |
| Pleochroism | Present, dichroism or trichroism, depending on heat treatment |
Tanzanite is the blue and violet variety of the mineral zoisite (a calcium aluminium hydroxyl sorosilicate), caused by small amounts of vanadium.[3] Tanzanite belongs to the epidote mineral group. Tanzanite is only found in Simanjiro District of Manyara Region in Tanzania, in a very small mining area approximately 7 km (4.3 mi) long and 2 km (1.2 mi) wide[4] near the Mererani Hills.[5]
Tanzanite is noted for its remarkably strong trichroism, appearing alternately blue, violet and burgundy depending on crystal orientation.[6] Tanzanite can also appear differently when viewed under different 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. In its rough state tanzanite is coloured a reddish brown to clear, and it requires heat treatment to remove the brownish "veil" and bring out the blue violet of the stone.[7]
The gemstone was given the name "tanzanite" by Tiffany & Co. after Tanzania, the country in which it was discovered. The scientific name of "blue-violet zoisite" was not thought to be sufficiently consumer friendly by Tiffany's marketing department, who introduced it to the market in 1968. In 2002, the American Gem Trade Association chose tanzanite as a December birthstone, the first change to their birthstone list since 1912.[8]
- ^ a b c d Anthony, John W.; Bideaux, Richard A.; Bladh, Kenneth W.; Nichols, Monte C., eds. (2001). Zoisite (PDF). Chantilly, Virginia: Mineralogical Society of America. Retrieved 9 April 2014.
- ^ Anthony, John W.; Bideaux, Richard A.; Bladh, Kenneth W.; Nichols, Monte C., eds. (1995). Handbook of Mineralogy, Vol. 2. Mineral Data Publishing. p. 901. ISBN 978-0-9622097-0-3.
- ^ King, Hobart M. "Tanzanite". Geology.com. Retrieved 3 December 2018.
The blue color of tanzanite is caused by small amounts of vanadium within the zoisite mineral structure.
- ^ "Introduction to tanzanite". tanzanitefoundation.com. 22 June 2017.
- ^ Briggs, Philip; McIntyre, Chris (2013). Tanzania Safari Guide: With Kilimanjaro, Zanzibar and the Coast. Bradt Travel Guides. p. 104. ISBN 978-1-84162-462-4.
- ^ E. Skalwold. "Pleochroism: trichroism and dichroism in gems". Nordskip.com. Archived from the original on 6 September 2012. Retrieved 29 August 2011.
- ^ "Study of Heat Treatment". Yourgemologist.com. International School of Gemology. Retrieved 29 August 2011.
- ^ "AGTA GTC to Issue Tanzanite Reports". JCK Magazine. 12 November 2007. Retrieved 31 December 2013 – via jckonline.com.