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Article: Tanzanite


General Information about Tanzanite

  • Tanzanite is a variety of the mineral zoisite.
  • It is a sorosilicate with the chemical formula (Ca(SiO)(Si)O(OH)) + (Cr,Sr).
  • Tanzanite belongs to the epidote mineral group.
  • It is known for its royal blue, indigo, and violet/purple color.
  • The crystal habit of tanzanite is prismatic with striations; it can also be massive to columnar.

Geology and Discovery of Tanzanite

  • Tanzanite was formed around 585 million years ago during the mid-Ediacaran Period.
  • It was formed due to plate tectonic activity and intense heat in the area that later became Mount Kilimanjaro.
  • Deposits of tanzanite are typically found in the hinge of isoclinal folds.
  • The mineral is located in a complex geological environment.
  • Tanzanite was discovered in January 1967 by Jumanne Mhero Ngoma.
  • Ngoma found the sparkling blue stones at the Mererani hills in the Kiteto district of Tanzania.
  • The gemstone was initially mistaken for olivine and dumortierite before being correctly identified as a variety of zoisite.
  • The name 'tanzanite' was given to the gemstone by Tiffany & Co. after Tanzania, the country of its discovery.

Commercial History and Mining Developments of Tanzanite

  • In July 1967, transparent fragments of tanzanite were found by Manuel de Souza near Mererani.
  • The gemstone was initially thought to be olivine or dumortierite.
  • The correct identification was made by the Gemological Institute of America and other mineralogists.
  • Tiffany & Co. renamed the gemstone as 'tanzanite' to capitalize on its rarity and single location.
  • In 1971, the Tanzanian government nationalized the tanzanite mines.
  • The Tanzanian government split the tanzanite mines into four sections: Blocks A, B, C, and D.
  • TanzaniteOne, a major player in the tanzanite market, obtained the lease for Block C.
  • Legislation was introduced in 2003 to ban the export of unprocessed tanzanite to India.
  • In 2010, the government banned the export of rough stones weighing more than one gram.
  • TanzaniteOne Mining Ltd. is owned by Richland Resources and had a record production in 2019.

Heat Treatment and Pleochroism in Tanzanite

  • Tanzanite forms as a brownish crystal and is trichroic, showing three colors concurrently: brown, blue, and violet.
  • Heating removes the brown or burgundy color component, producing a stronger violet-blue color and making the stone dichroic.
  • Some gem-quality tanzanite can heat to a green primary hue, but it is seldom of interest to commercial buyers.
  • Heat treatment is usually carried out at temperatures between 370 and 390°C for 30 minutes.
  • Tanzanite found close to the surface in the early days of discovery may not require heat treatment.
  • Tanzanite is a pleochroic gemstone, meaning it appears to have multiple colors based on the angle of light.
  • Most tanzanite appears blue when viewed from one direction but can vary from violet to red when seen from a different angle.
  • The cutting process can be challenging due to the problem of selecting the perfect color.
  • The finished color of the gemstone depends on how the table cut reflects the light.

Imitation and Cobalt-coated Tanzanite

  • Tanzanite has never been successfully synthesized in a laboratory, so all genuine tanzanite is naturally occurring.
  • Tanzanite has been imitated using materials such as cubic zirconia, synthetic spinel, yttrium aluminium garnet, and colored glass.
  • Genuine tanzanite can be distinguished from imitations using a dichroscope, as only tanzanite will appear doubly refractive.
  • Synthetic forsterite has been sold as tanzanite but can be distinguished through refractometer readings and Hanneman filter tests.
  • Lower grades of tanzanite are occasionally enhanced using a layer of cobalt, but the practice is considered deceptive unless disclosed.

Tanzanite Data Sources

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