Tanzanite

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Tanzanite
Zoïsite (Tanzanite).jpg
General
CategorySorosilicate: zoisite variety
Formula
(repeating unit)
(Ca2Al3(SiO4)(Si2O7)O(OH)) + (Cr,Sr)
Strunz classification09.BG.10
Crystal systemOrthorhombic[1]
Identification
ColorRoyal blue, indigo, violet/purple
Crystal habitPrismatic crystals with striations; massive to columnar[1][2]
Twinningpenetration twins
CleavagePerfect {010}, imperfect {100}[1]
FractureUneven to conchoidal[1]
Mohs scale hardness6.5
LusterVitreous, pearly on cleavage surfaces
StreakWhite or colorless
Specific gravity3.10–3.38
Optical propertiesbiaxial positive
Refractive index1.69–1.70
Birefringence0.006–0.018
PleochroismPresent, 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 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 [sw] 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 colored 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]

Geology[edit]

Tanzanite was formed around 585 million years ago during the mid-Ediacaran Period by massive plate tectonic activity and intense heat in the area that would later become Mount Kilimanjaro. The mineral is located in a relatively complex geological environment. Deposits are typically found in the "hinge" of isoclinal folds.[9]

Commercial history[edit]

Craft work on tanzanite

In July 1967, Manuel de Souza, a Goan tailor and part-time gold prospector living in Arusha (Tanzania), found transparent fragments of blue and blue-purple gem crystals on a ridge near Mirerani, some 40 km (25 mi) southeast of Arusha.[10][11][12] He assumed that the mineral was olivine (peridot) but, after soon realizing it was not, he concluded it was "dumortierite" (a blue non-gem mineral). Shortly thereafter, the stones were shown to John Saul, a Nairobi-based consulting geologist and gemstone wholesaler who was then mining aquamarine in the region around Mount Kenya. Saul, who later discovered the famous ruby deposits in the Tsavo area of Kenya, eliminated dumortierite and cordierite as possibilities, and sent samples to his father, Hyman Saul, vice president at Saks Fifth Avenue in New York. Hyman Saul brought the samples across the street to the Gemological Institute of America who correctly identified the new gem as a variety of the mineral zoisite. Correct identification was also made by mineralogists at Harvard University, the British Museum, and Heidelberg University, but the very first person to get the identification right was Ian McCloud, a Tanzanian government geologist based in Dodoma.[13][14]

Scientifically called "blue zoisite", the gemstone was renamed as tanzanite by Tiffany & Co., who wanted to capitalize on the rarity and single location of the gem, and thought that "blue zoisite" (which might be pronounced like "blue suicide") would not sell well.[15] Tiffany's original campaign advertised that tanzanite could now be found in two places: "in Tanzania and at Tiffany's".

From 1967, an estimated two million carats of tanzanite were mined in Tanzania before the mines were nationalized by the Tanzanian government in 1971.

Multicolored tanzanite crystal
Untreated tanzanite gemstone

Tanzanite mining developments[edit]

In 1990, the Tanzanian government split the tanzanite mines into four sections: Blocks A, B, C and D. Blocks A and C were awarded to large operators, while Blocks B and D were reserved for the local miners. In 2005 the government renewed the lease of Block C mine to TanzaniteOne, who paid US$40 million for their lease and mining license.[citation needed]

In June 2003, the Tanzanian government introduced legislation banning the export of unprocessed tanzanite to India. (Like many gemstones, most tanzanite is cut in Jaipur.) The reason for the ban is to attempt to spur development of local processing facilities, thereby boosting the economy and recouping profits. This ban was phased in over a two-year period, until which time only stones over 0.5 grams were affected.[citation needed] In 2010, the government of Tanzania banned the export of rough stones weighing more than one gram.[16]

TanzaniteOne Mining Ltd is owned by Richland Resources, but a 2010 law in Tanzania required them to cede 50% ownership of their mining license to the Tanzanian State Mining Company (Stamico). Production in 2011 amounted to 2,400,000 carats (480 kg; 1,100 lb), earning them $24 million.[17]

Following the construction of a 24 km (15 mi) perimeter wall around the mines, to improve security and prevent smuggling,[18] production rose from 147.7 kg (325.6 lb) in 2018 to a record 781.2 kg (1,722 lb) in 2019.[19]

On 24 June 2020, a new record for the world's largest rough tanzanite was set after a small-scale miner, Mr. Saniniu Laizer, mined two stones of 9.72 kg (21.4 lb) and 5.1 kg (11 lb) and sold them to the Government of Tanzania through Ministry of Mining for 7.74 bn Tanzanian shillings (US$3.35 mn)[20] surpassing a record of 16,839 carats (3.37 kg; 7.42 lb) stone mined by TanzaniteOne in 2005.[21]

Total reserves of tanzanite are estimated at 109,000,000 carats (21,800 kg; 48,100 lb), according to a report published in 2018.[22] Block C, by far the largest site, has been estimated at 87,100,000 carats (17,400 kg; 38,400 lb) with a Life of Mine (LOM) expected to last until the 2040s.[23]

Factors affecting value: grading[edit]

There is no universally accepted method of grading colored gemstones. TanzaniteOne, a major commercial player in the tanzanite market, through its non-profit subsidiary, the Tanzanite Foundation,[24] has introduced its own color-grading system.[25] The new system's color-grading scales divide tanzanite colors into a range of hues, between bluish-violet, indigo and violetish-blue.

The normal primary and secondary hues in tanzanite are blue and violet. Untreated tanzanite is a trichroic gemstone, meaning that light that enters this anisotropic crystal gets refracted on different paths, with different color absorption on each of the three optical axes. As a result of this phenomenon, a multitude of colors have been observed in various specimens: shades of purple, violet, indigo, blue, cyan, green, yellow, orange, red and brown. After heating, tanzanite becomes dichroic. The dichroic colors range from violet through bluish-violet to indigo and violetish-blue to blue.[26]

Clarity grading in colored gemstones is based on the eye-clean standard, that is, a gem is considered flawless if no inclusions are visible with the unaided eye (assuming 20/20 vision).[26] The Gemological Institute of America classifies tanzanite as a Type I gemstone, meaning it is normally eye-clean. Gems with eye-visible inclusions will be traded at deep discounts.

Heat treatment[edit]

A rough sample of tanzanite.

Tanzanite forms as a brownish crystal and is trichroic, which means it shows three colors – brown, blue and violet – concurrently. Heating, either underground naturally by metamorphic processes, or artificially, removes the brown or burgundy color component to produce a stronger violet-blue color and makes the stone "dichroic", which means it only reflects blue and violet.[27] Rarely, gem-quality tanzanite will heat to a green primary hue, almost always accompanied by a blue or violet secondary hue. These green tanzanite have some meaningful value in the collector market, but are seldom of interest to commercial buyers.[28]

Heat-treating in a furnace is usually carried out at between 370 to 390 °C (698 to 734 °F) for 30 minutes. The stones should not have any cracks or bubbles, as they could shatter or the cracks/ bubble could increase in size during furnace heating.[29]

Some stones found close to the surface in the early days of the discovery (in an area now called D block) were gem-quality blue without the need for heat treatment – probably the result of a wild fire in the area which heated the stones underground. This gave rise to the idea that "D block" stones were more desirable than tanzanite found in other areas of the small tanzanite mining area.

Since heat treatment is universal, it has no effect on price, and finished gems are assumed to be heat-treated. Gemological Institute of America states that the source of heating is gemologically undetectable, but is assumed because of its prevalence.[30]

Tanzanite may be subjected to other forms of treatment as well. Recently, coated tanzanites were discovered and tested by the AGTA and AGL laboratories.[31] A thin layer containing cobalt, determined by X-ray fluorescence, had been applied to improve the color. It was noted that "coatings in particular are not considered permanent", and in the United States are required to be disclosed at the point of sale.

See also[edit]

References[edit]

  1. ^ a b c d Anthony, John W.; Bideaux, Richard A.; Bladh, Kenneth W.; Nichols, Monte C., eds. (2001). "Zoisite" (PDF). Handbook of Mineralogy. Chantilly, Virginia: Mineralogical Society of America. Retrieved 9 April 2014.
  2. ^ 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.
  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.
  4. ^ "Introduction to tanzanite". tanzanitefoundation.com. 22 June 2017.
  5. ^ 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.
  6. ^ E. Skalwold. "Pleochroism: trichroism and dichroism in gems". Nordskip.com. Archived from the original on 6 September 2012. Retrieved 29 August 2011.
  7. ^ "Study of Heat Treatment". Yourgemologist.com. International School of Gemology. Retrieved 29 August 2011.
  8. ^ "AGTA GTC to Issue Tanzanite Reports". JCK Magazine. 12 November 2007. Retrieved 31 December 2013 – via jckonline.com.
  9. ^ "Tanzanite Mining". TanzaniteOne. 2018. Retrieved 25 June 2020.
  10. ^ Saul, John. "The discovery of Tanzanite". Jeweller. Retrieved 25 June 2020.
  11. ^ "The Discovery of Tanzanite". Gemporia. Gemporia Limited. 30 June 2019. Retrieved 25 June 2020.
  12. ^ Luhr, James (2003). Earth. New York, NY: DK Publishing, Inc. p. 73. ISBN 0-7894-9643-7.
  13. ^ "Merelani, Tanzania". The Mineralogical Record. Archived from the original on 7 August 2010 – via minrec.org.
  14. ^ "Tanzanite: Its discovery and early days". ICA's InColor Magazine. Summer 2007.
  15. ^ "Tanzanite". Gemstone.org. Archived from the original on 12 September 2008. Retrieved 29 August 2011.
  16. ^ "Export ban on tanzanite in Africa leaves Jaipur gems sector in lurch". The Times of India. 3 October 2013. Retrieved 31 December 2013.
  17. ^ Muchira, John (24 May 2013). "TanzaniteOne cedes 50% stake to State as new law takes effect". Mining Weekly.
  18. ^ "Tanzania orders wall built around tanzanite mines to end illegal trade". Reuters.com. 20 September 2017. Retrieved 27 June 2020.
  19. ^ "Tanzania: Tanzanite Sales, Revenue Soar As Smuggling Is Checked". AllAfrica.com. 16 May 2019. Retrieved 25 June 2020.
  20. ^ "Artisanal miner in Tanzania finds large rare gemstones worth $3.3 million". Reuters.com. 24 June 2020. Retrieved 24 June 2020.
  21. ^ Cobley, Mark (3 August 2005). "World's Biggest Tanzanite Gem Found Near Kilimanjaro (Update3)". Bloomberg. Archived from the original on 24 September 2015. Retrieved 31 December 2013.
  22. ^ "National Environment Statistics Report - Tanzania Mainland" (PDF). National Bureau of Statistics (NBS) (Tanzania). June 2018. Retrieved 27 June 2020.
  23. ^ Luvanda, H. E. Baraka H. (10 October 2018). "Presentation of Tanzania High Commissioner Of Tanzania Minerals And Metals Outlook, 2030". The High Commission of the United Republic of Tanzania. Retrieved 27 June 2020.
  24. ^ "The Tanzanite Foundation". The Tanzanite Foundation. 22 April 2013. Archived from the original on 9 February 2005. Retrieved 26 November 2013.
  25. ^ Roskin, Gary (May 2005). "Tanzanite Transformed: TanzaniteOne Introduces Quality Grading, Pricing Changes, and a Sight System". JCK Magazine. Retrieved 4 May 2018.
  26. ^ a b Richard W. Wise (31 December 2005). Secrets of the Gem Trade: The Connoisseur's Guide to Precious Gemstones. Brunswick House Press. pp. 35, 220. ISBN 978-0-9728223-8-1.
  27. ^ Weldon, Robert. "An introduction to gem treatments". gia.edu. Gemological Institute of America, Inc.
  28. ^ "Tanzanite Gemological Information". gemsociety.org.
  29. ^ Roskin, Gary (February 2005). "Natural-Color Tanzanite". JCK Magazine – via jckonline.com.
  30. ^ "Featured gemstone: tanzanite". GIA Library. Gemological Institute of America, Inc. Archived from the original on 13 May 2008.
  31. ^ "Coatings: a new Tanzanite treatment uncovered". farlang.com. Farlang's Gem and Diamond Foundation; Gemological Association of Great Britain; German Gemological Association; Netherlands Gemological Laboratory. 25 May 2008.

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