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Tanzanite

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Tanzanite
Calcite-Zoisite-denv08-08a.jpg

Tanzanite on calcite from the Merelani Hills (Mererani), Lelatema Mountains, Arusha, Tanzania (size: 6.5 cm × 5.7 cm × 4.2 cm)

General and classification
Chemical formula Ca2Al3(SiO4)3OH
Mineral class
(and division, if applicable)
see zoisite
Similar minerals Amethyst, Cordierite, Lazulite, Sapphire, Spinel
Crystallographic data
Crystal system orthorhombic
Crystal class; Symbol orthorhombic-dipyramidal; 2/m 2/m 2/m
Physical properties
Mohs hardness 6.5 to 7
Density (g/cm3) 3.2 to 3.4
Fissility completely after {100}
Fracture; Tenacity uneven, brittle
Color purple, sapphire blue, reddish violet, greenish yellow
Line colour white
Transparency see-through
Glamour Glass luster, pearlescent luster on cleavage surfaces[1]
Crystal Optics
Refractive indices = 1.691[1]
= 1.692[1]
= 1.700[1]
Birefringence δ = 0,009[2]
Visual character biaxial positive[2]
Pleochroism very strong: purple, blue and brown or yellow[2]
Other features
Chemical behaviour not acid-proof

Tanzanite is a blue variety of the grey to greenish mineral zoisite. Like the pink variety thulite, it is used exclusively as a gemstone.

Etymology and history

The first finds of the transparent, purple-bluish to purple-colored tanzanite were made in 1967 in northern Tanzania. The stone first became known through the New York jeweler Tiffany. He named it after its place of origin, tanzanite, because the name zoisite reminded him too much of the English word for suicide.

Formation and sites

Tanzanite forms predominantly from hydrothermal solutions in veins and fissures of gneisses.

The only mineable and commercially exploited occurrence of high quality tanzanite crystals is still in the Gilewy Hills near Arusha in Tanzania.[1][2] However, current finds rarely reach the outstanding quality of earlier years.[3]

Tanzanite finds have also been reported from the Hindu Kush region of Pakistan, but these stones do not attain the color quality or purity of those from Tanzania, nor can they be transformed into the coveted blue-violet color by firing.[3][4]

Use as a gemstone

Tanzanite navette

Usually, tanzanite occurs in sizes between 0.5 and 10 carats. Rarely, larger crystals and tanzanite cat’s eyes are also found. Two of the largest tanzanites known to date were found in an unnamed pit in Tanzania. The two gems are each 30 cm long and about 10 cm thick. Weighing a total of more than 14 kg (9.27 kg and 5.103 kg), they fetched a sum of 7.74 billion Tanzania shillings (US$3.35 million) when sold to the local mining ministry.[5]

Previously, the 22 centimeter high and 11,000 carat “Kilimanjaro” was considered the largest known tanzanite crystal.[6]

Very pronounced is the multicolor (pleochroism) of tanzanite in the colors purple, blue and brown or yellow.[2] The larger the stone, the more intense the color. Thanks to its unusual radiance and with the help of the New York jeweler Tiffany, it quickly advanced to become one of the most sought-after gemstones in the world.[7]

Because of its high transparency, the mineral is very popular as a gemstone and fetches high prices. Due to its pronounced cleavage in one direction, tanzanite reacts sensitively not only to compressive stresses, as they occur in grinding and barrel work, but also to uneven thermal stresses or rapid temperature changes. Even removing the object to be repaired from the soldering carbon or placing the piece on the soldering sheet or board to cool can cause cracks in the stone.[8] The gemstone also does not tolerate cleaning in an ultrasonic device. It also reacts sensitively to acids or acid mixtures.

Since tanzanite has a relatively low Mohs hardness of 6.5 to 7 for gemstones, which is lower than that of quartz (7), the sensitivity of the stone must also be taken into account when wearing tanzanite jewelry. Otherwise, rough handling may quickly cause scratches.

Manipulations and imitations

A large proportion of the tanzanites on the market are produced by firing at about 400 to 500 °C[2] from grey-brown to yellow-brown zoisites. Firing also enhances the blue color of weakly colored stones. However, tanzanite is also imitated by coloured glass (cheap variety) or doublets.

Synthetic tanzanite is so far unknown. The “synthetic tanzanite” occasionally offered at mineral exchanges is mostly synthetic forsterite.[9]

See also

  • List of minerals
  • List of mineral gemstones and precious stones

Literature

  • Walter Schumann: Gemstones and Precious Stones. All types and varieties. 1900 singles. 16., revised edition. BLV Verlag, Munich 2014, ISBN 978-3-8354-1171-5, p. 176.
  • Bernhard Bruder: Embellished Stones. The recognition of imitations and manipulations in gemstones and minerals. Neue Erde, Saarbrücken 2005, ISBN 3-89060-079-4, p. 101.

Web links

Commons: Tanzanite– Collection of images, videos and audio files

Individual references

  1. a b c d e
    Jaroslav Bauer, Vladimír Bouška: Edelsteinführer. Verlag Werner Dausien, Hanau/Main 1993, ISBN 3-7684-2206-2, p. 206.
  2. a b c d e f
    Walter Schumann: Gemstones and Precious Stones. All types and varieties. 1900 singles. 16., revised edition. BLV Verlag, Munich 2014, ISBN 978-3-8354-1171-5, p. 176.
  3. a b
    Gemstone Encyclopedia – Tanzanite.In: carat-online.at. Retrieved 25 June 2020.

  4. Edelsteinlabor Dieter Pschichholz: Tanzanite (PDF 786 kB)(Memento of April 7, 2014 in the Internet Archive)

  5. Jason Burke:Tanzanian mine owner celebrates discovery of $3.3m gemstones.In: theguardian.com. The Guardian, 25 June 2020, accessed 25 June 2020.

  6. Matthias Benz:Merelani Mine – The site of the world famous tanzanites.World of Crystals, June 1, 2013, accessed June 25, 2020.

  7. ICA Gem Bureau Idar-Oberstein, Tanzanite(Memento of 30 November 2016 in the Internet Archive)

  8. Gemstone Etiquette by Prof. Leopold Rössler – Tanzanite.In: beyars.com. Retrieved 25 June 2020.

  9. Bernhard Bruder: Embellished Stones. The recognition of imitations and manipulations in gemstones and minerals. Neue Erde, Saarbrücken 2005, ISBN 3-89060-079-4, p. 101.