View source for Tanzanite

{{Use dmy dates|date=June 2020}}
{{short description|Blue to purple variety of the mineral zoisite}}
{{hatnote group|
{{distinguish|Tasmanite}}
{{for|the exoplanet|WASP-71b}}
}}
{{Infobox mineral
|category      = [[Sorosilicate]]: zoisite variety
|boxwidth      = 
|boxbgcolor    = #cba0fa 
|image         = Zoïsite (Tanzanite).jpg
|imagesize     = 260px
|caption       = 
|strunz        = 09.BG.10
|formula       = {{chem2|[Ca2Al3(SiO4)(Si2O7)O(OH)] + (Cr,Sr)}} 
|color         = Royal blue, indigo, violet/purple
|habit         = Prismatic crystals with striations; massive to columnar<ref name=hom>{{cite encyclopedia|year= 2001|editor1-first= John W.|editor1-last= Anthony|editor2-first= Richard A.|editor2-last= Bideaux|editor3-first= Kenneth W.|editor3-last= Bladh|editor4-first= Monte C.|editor4-last= Nichols|publisher= Mineralogical Society of America|location= Chantilly, Virginia|url= http://www.handbookofmineralogy.com/pdfs/zoisite.pdf|title= Zoisite|access-date= 9 April 2014|archive-date= 9 May 2020|archive-url= https://web.archive.org/web/20200509111644/http://www.handbookofmineralogy.com/pdfs/zoisite.pdf|url-status= dead}}</ref><ref>{{cite book|editor1-first=John W.|editor1-last= Anthony|editor2-first= Richard A.|editor2-last= Bideaux|editor3-first= Kenneth W.|editor3-last=Bladh|editor4-first= Monte C.|editor4-last= Nichols|title= Handbook of Mineralogy, Vol. 2|year= 1995|publisher= Mineral Data Publishing|isbn= 978-0-9622097-0-3|page=901}}</ref>
|system        = [[Orthorhombic]]<ref name=hom/>
|symmetry      = ''Pnma'' (no. 62)
|twinning      = penetration twins
|cleavage      = Perfect&nbsp;{010}, imperfect&nbsp;{100}<ref name=hom/>
|fracture      = Uneven to conchoidal<ref name=hom/>
|mohs          = 6.5
|luster        = Vitreous, pearly on cleavage surfaces
|refractive    = 1.69–1.70
|opticalprop   = biaxial positive
|birefringence = 0.006–0.018
|pleochroism   = Present, [[dichroism]] or [[trichroism]], depending on heat treatment
|streak        = White or colorless
|gravity       = 3.10–3.38
|density       = 
|fusibility    = 
|diagnostic    = 
|solubility    = 
|diaphaneity   = 
|other         = 
}}
'''Tanzanite''' is the blue and violet variety of the mineral [[zoisite]] (a calcium aluminium [[hydroxyl]] [[sorosilicate]]), caused by small amounts of [[vanadium]].<ref>{{cite web|last1=King|first1=Hobart M.|title=Tanzanite|url=https://geology.com/gemstones/tanzanite/|website=Geology.com|access-date=3 December 2018|quote=The blue color of tanzanite is caused by small amounts of vanadium within the zoisite mineral structure.}}</ref> 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 {{cvt|7|km}} long and {{cvt|2|km}} wide<ref>{{Cite web |date=22 June 2017 |title=Introduction to Tanzanite |url=http://www.tanzanitefoundation.com/about-tanazanite/introduction-to-tanzanite/ |url-status=usurped |archive-url=https://web.archive.org/web/20170820140341/http://www.tanzanitefoundation.com/about-tanazanite/introduction-to-tanzanite/ |archive-date=20 August 2017 |website=tanzanitefoundation.com}}</ref> near the Mererani Hills.<ref name= "BriggsMcIntyre2013">{{cite book|last1=Briggs|first1=Philip|last2=McIntyre|first2=Chris|title=Tanzania Safari Guide: With Kilimanjaro, Zanzibar and the Coast|url= https://books.google.com/books?id=Fh4OFXXNkiIC&pg=PA104|year=2013|publisher=Bradt Travel Guides|isbn=978-1-84162-462-4|page=104}}</ref>

Tanzanite is noted for its remarkably strong [[trichroism]], appearing alternately blue, violet and burgundy depending on [[crystal]] orientation.<ref>{{cite web|author=E. Skalwold|url=http://www.nordskip.com/pleochroism.html|title=Pleochroism: trichroism and dichroism in gems|work=Nordskip.com|access-date=29 August 2011|url-status=dead|archive-url=https://archive.today/20120906225019/http://www.nordskip.com/pleochroism.html|archive-date=6 September 2012}}</ref> 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 light|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.<ref>{{cite web|url= http://www.yourgemologist.com/heattreatment.html|work= Yourgemologist.com|publisher= International School of Gemology|title= Study of Heat Treatment|access-date=29 August 2011}}</ref> 

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 Society|American Gem Trade Association]] chose tanzanite as a December [[birthstone]], the first change to their birthstone list since 1912.<ref>{{cite news|title= AGTA GTC to Issue Tanzanite Reports|work= JCK Magazine|date= 12 November 2007|via= jckonline.com|url= http://www.jckonline.com/2007/11/12/agta-gtc-to-issue-tanzanite-reports|access-date=31 December 2013}}</ref>

==Geology==
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 [[Fold (geology)|isoclinal folds]].<ref>{{cite news|url=https://www.tanzaniteone.com/tanzanite-mining|title=Tanzanite Mining|year=2018|access-date=25 June 2020|work=TanzaniteOne}}</ref>

==Discovery==
There are many accounts of the discovery of tanzanite, but only one recognised by the government of Tanzania. In January 1967, [[Jumanne Mhero Ngoma]] (originally from [[Same District, Kilimanjaro]]) stumbled upon the sparkling blue stones at the Mererani hills in the Kiteto district of the then [[Arusha Region]] (presently [[Manyara Region]]). He was issued with a certificate of recognition three years later by the then President [[Julius Nyerere]] and a financial reward of Tsh 50,000 for his efforts. In 1984, he was issued with a certificate for scientific discovery by the [[Tanzania Commission for Science and Technology]].<ref>{{Cite news|title=Mgunduzi wa madini Tanzanite afariki dunia|language=sw|work=BBC News Swahili|url=https://www.bbc.com/swahili/habari-47063299|access-date=2023-09-26}}</ref><ref>{{Cite web|title=Tanzanian gemstone finder living in poverty|url=https://www.aa.com.tr/en/africa/tanzanian-gemstone-finder-living-in-poverty/921643|access-date=2023-09-26|website=www.aa.com.tr}}</ref>

==Commercial history==
[[File:Craft work on tanzanite.jpg|thumb|left|Craft work on tanzanite]]

In July 1967, Manuel de Souza, a [[Goan]] tailor and part-time gold prospector living in [[Arusha]], found transparent fragments of blue and blue-purple gem crystals on a ridge near Mererani, some {{convert|40|km|abbr=on}} southeast of Arusha.<ref>{{cite web|last1=Saul|first1=John|title=The discovery of Tanzanite|url=https://www.jewellermagazine.com/Article/220/The-discovery-of-tanzanite|website=Jeweller |date=16 August 2009 |access-date=25 June 2020}}</ref><ref>{{cite web|title=The Discovery of Tanzanite|url=https://www.gemporia.com/en-gb/gemology-hub/article/27/the-discovery-of-tanzanite/|website=Gemporia|publisher=Gemporia Limited|access-date=25 June 2020|date=30 June 2019}}</ref><ref>{{cite book|last1=Luhr|first1=James|title=Earth|date=2003|publisher=DK Publishing, Inc.|location=New York, NY|isbn=0-7894-9643-7|page=[https://archive.org/details/earth0000unse_o0d8/page/73 73]|url=https://archive.org/details/earth0000unse_o0d8/page/73}}</ref> 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 [[Beryl|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 took the samples to the [[Gemological Institute of America]] which 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 first person to get the identification right was Ian McCloud, a Tanzanian government geologist based in [[Dodoma]].<ref>{{cite web|url=http://www.minrec.org/detail.asp?id=281|archive-url=https://web.archive.org/web/20100807215148/http://www.minrec.org/detail.asp?id=281|url-status=dead|archive-date=7 August 2010|work=The Mineralogical Record|title=Merelani, Tanzania|via=minrec.org}}</ref><ref>{{cite magazine|title=Tanzanite: Its discovery and early days|magazine=ICA's InColor Magazine|date=Summer 2007}}</ref>

Scientifically called "blue zoisite", the gemstone was renamed as ''tanzanite'' by Henry B. Platt, a great-grandson of [[Louis Comfort Tiffany]] and a vice president of Tiffany & Co.,<ref>{{cite journal|author=[[Eugenia Sheppard]]|title=Inside Fashion: Something new at Tiffany's is Tanzanian blue gem|url=https://newspapers.com/article/the-herald-sun-tanzanite/135491185/|journal=[[w:The Herald-Sun (Durham, North Carolina)|Durham Morning Herald]]|edition=final|others=Section B|location=Durham, N.C.|publisher=The Durham Herald Co.|date=3 October 1968|page=2B|oclc=9519676}}</ref> who wanted to capitalize on the rarity and single location of the gem and thought that "blue zoisite" (zoi′sīt) would not sell well.<ref>{{cite web|url=http://www.gemstone.org/gem-by-gem/english/tanzanite.html|archive-url=https://web.archive.org/web/20080912134702/http://www.gemstone.org/gem-by-gem/english/tanzanite.html|archive-date=12 September 2008|title=Tanzanite|website=Gemstone.org|access-date=29 August 2011}}</ref> 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 [[Carat (mass)|carats]] of tanzanite were mined in Tanzania before the mines were nationalized by the Tanzanian government in 1971.

[[File:Trichroic Tanzanite Crystal - violet & burgundy.jpg|thumb|upright|Multicolored tanzanite crystal]]
[[File:Trichroic Tanzanite Gem - blue, violet & purple.jpg|thumb|upright|Untreated tanzanite gemstone]]

===Tanzanite mining developments===
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|date=April 2015}}

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|date=August 2008}} In 2010, the government of Tanzania banned the export of rough stones weighing more than one gram.<ref>{{cite news|title=Export ban on tanzanite in Africa leaves Jaipur gems sector in lurch|date=3 October 2013|url=https://timesofindia.indiatimes.com/city/jaipur/Export-ban-on-tanzanite-in-Africa-leaves-Jaipur-gems-sector-in-lurch/articleshow/23435984.cms|archive-url=https://web.archive.org/web/20131229000508/http://articles.timesofindia.indiatimes.com/2013-10-03/jaipur/42663354_1_tanzanite-jaipur-gem-stones|url-status=live|archive-date=29 December 2013|work=[[The Times of India]]|access-date=31 December 2013}}</ref>

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 {{convert|2,400,000|carat|kg lb|sigfig=2}}, earning them $24&nbsp;million.<ref>{{cite news|title=TanzaniteOne cedes 50% stake to State as new law takes effect|last=Muchira|first=John|url=http://www.miningweekly.com/article/Tanzania-2013-05-16|work=Mining Weekly|date=24 May 2013}}</ref>

Following the construction of a {{cvt|24|km|sigfig=2}} perimeter wall around the mines, to improve security and prevent smuggling,<ref>{{cite news|url=https://www.reuters.com/article/us-tanzania-mining-wall/tanzania-orders-wall-built-around-tanzanite-mines-to-end-illegal-trade-idUSKCN1BV1YF|title=Tanzania orders wall built around tanzanite mines to end illegal trade|date=20 September 2017|access-date=27 June 2020|work=Reuters.com}}</ref> production rose from {{cvt|147.7|kg|sigfig=4}} in 2018 to a record {{cvt|781.2|kg|sigfig=4}} in 2019.<ref>{{cite news|url=https://allafrica.com/stories/201905160543.html|work=AllAfrica.com|title=Tanzania: Tanzanite Sales, Revenue Soar As Smuggling Is Checked|date=16 May 2019|access-date=25 June 2020}}</ref>

On 24 June 2020, artisanal miner Saniniu Laizer unearthed two rough tanzanite stones, weighing '''11.14 kg''' (55,700 carats) in total, which he sold to the Tanzanian government’s Ministry of Mining for TSh 7.74 billion (US $3.35 million), breaking the previous record set in 2005 by a 16,839‑carat stone<ref>{{cite news|title=World's Biggest Tanzanite Gem Found Near Kilimanjaro (Update3)|last=Cobley|first=Mark|date=3 August 2005|work=Bloomberg|url=https://www.bloomberg.com/apps/news?pid=newsarchive&sid=aCeSwFRz6I.8|access-date=31 December 2013|archive-url=https://web.archive.org/web/20150924144505/https://www.bloomberg.com/apps/news?pid=newsarchive&sid=aCeSwFRz6I.8|archive-date=24 September 2015}}</ref>

Total reserves of tanzanite are estimated at {{convert|109,000,000|carat|kg lb|sigfig=3}}, according to a report published in 2018.<ref>{{cite news|url=https://unstats.un.org/unsd/environment/Compendia/United%20Republic%20of%20Tanzania,%20National%20Environment%20Statistics%20Report,%202017.pdf|title=National Environment Statistics Report - Tanzania Mainland|date=June 2018|access-date=27 June 2020|work=National Bureau of Statistics (NBS) (Tanzania)}}</ref> Block C, by far the largest site, has been estimated at {{convert|87,100,000|carat|kg lb|sigfig=3}} with a Life of Mine (LOM) expected to last until the 2040s.<ref>{{cite web|url=https://www.in.tzembassy.go.tz/resources/view/presentation-by-h.e-baraka-h-luvanda-tanzania-high-commissioner-of-tanzania-minerals-and-metals-outlook-2030|title=Presentation of Tanzania High Commissioner Of Tanzania Minerals And Metals Outlook, 2030|first=H. E. Baraka H.|last=Luvanda|date=10 October 2018|access-date=27 June 2020|work=The High Commission of the United Republic of Tanzania}}</ref>

==Factors affecting value: grading==
There is no universally accepted method of grading coloured gemstones. TanzaniteOne, a major commercial player in the tanzanite market, through its non-profit subsidiary, the Tanzanite Foundation,<ref>{{cite web|url=http://tanzanitefoundation.org/|archive-url=https://web.archive.org/web/20050209115856/http://www.tanzanitefoundation.org/|url-status=dead|archive-date=9 February 2005|title=The Tanzanite Foundation|publisher=The Tanzanite Foundation|date=22 April 2013|access-date=26 November 2013}}</ref> has introduced its own color-grading system.<ref>{{cite news|title=Tanzanite Transformed: TanzaniteOne Introduces Quality Grading, Pricing Changes, and a Sight System|work=JCK Magazine|last=Roskin, Gary|date=May 2005|url=https://www.jckonline.com/magazine-article/tanzanite-transformed-tanzaniteone-introduces-quality-grading-pricing-changes-and-a-sight-system/|access-date=4 May 2018}}</ref> The new system's colour-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 colour 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 [[Dichroism|dichroic]]. The dichroic colours range from violet through bluish-violet to indigo and violetish-blue to blue.<ref name="Wise2005">{{cite book|author=Richard W. Wise|title=Secrets of the Gem Trade: The Connoisseur's Guide to Precious Gemstones|pages=35,220|date=31 December 2005|publisher=Brunswick House Press|isbn=978-0-9728223-8-1}}</ref>

Clarity grading in coloured 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).<ref name="Wise2005"/> 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===
[[File:Tansanit nature.jpg|thumb|left|A rough sample of tanzanite.]]

Tanzanite forms as a brownish crystal that concurrently shows three colours – brown, blue, and violet – hence it is a ''"[[trichroic]]"'' mineral. Heating, either by natural metamorphic processes underground, or artificial industrial processes in a furnace, removes the brown or burgundy colour component to produce a stronger violet-blue color and makes the stone only reflects blue and violet, hence ''"[[dichroic]]"''.<ref>{{cite web |last=Weldon |first=Robert |date=c. 2002  |title=An introduction to gem treatments |publisher=[[Gemological Institute of America]] |url=http://www.gia.edu/gem-treatment |via=gia.edu }}</ref> 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.<ref>{{cite web |title=Tanzanite jewelry and gemstone information |website=gemsociety.org |url=http://www.gemsociety.org/article/tanzanite-jewelry-and-gemstone-information/ }}</ref>

Heat-treating in a furnace is usually carried out at between {{cvt|370|–|390|C}} for 30&nbsp;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.<ref>{{cite magazine |last=Roskin|first=Gary |date=February 2005 |title=Natural-color tanzanite |magazine=JCK Magazine |url=https://www.jckonline.com/magazine-article/natural-color-tanzanite/ |via=jckonline.com }}</ref>

Some stones found close to the surface in the early days of the discovery (in an area now called "D&nbsp;block") were gem-quality blue without the need for heat treatment, probably the result of a wildfire in the area which heated the stones underground. This gave rise to the idea that "D&nbsp;block" stones were more desirable than tanzanite found in other parts 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.<ref name=giaguide>{{cite web |title=Tanzanite |department=Featured gemstone |series=[[Gemological Institute of America|GIA]] Library (gia.edu/library) |publisher=[[Gemological Institute of America]] |url=http://www.gia.edu/library/4476/tanzanite.cfm |url-status=dead |archive-url=https://web.archive.org/web/20080513024727/http://www.gia.edu/library/4476/tanzanite.cfm |archive-date=13 May 2008 }}</ref>

Tanzanite may be subjected to other forms of treatment as well. Recently, coated tanzanites were discovered and tested by the AGTA and AGL laboratories.<ref name=Tanzanite-coating>{{cite web |title=Coatings: A new Tanzanite treatment uncovered |date=25 May 2008 |publisher=Farlang's Gem and Diamond Foundation; [[Gemmological Association of Great Britain]]; [[German Gemmological Association]]; Netherlands Gemological Laboratory |website=farlang.com |url=http://www.farlang.com/art/2008-05-25.1596715347 }}</ref> A thin layer containing [[cobalt]], determined by [[X-ray fluorescence]], had been applied to improve the colour. 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.

===Pleochroism in tanzanite===
Tanzanite has the physical property that it appears to have multiple colours based on the angle of the light hitting the stone, called ''[[pleochroism]]''. Most tanzanites are blue when viewed from one direction but can vary from violet to red when seen from a different angle.

The physical characters can make cutting process difficult due to the problem of selecting the 'perfect' color. The finished colour of the gemstone will vary depending on how the table cut reflects the light.<ref>{{cite web |title=Pleochroism effect |department=Tanzanite characteristics |date=21 July 2020 |website=starlanka.com |url=https://www.starlanka.com/journal/tanzanite-gemstone-royal-gemstone/ }}</ref>

==Imitation and cobalt-coated tanzanite==
{{As of|2020}}, tanzanite has never been successfully synthesized in a laboratory, so all genuine tanzanite is naturally occurring. However, because of its rarity and market demand, tanzanite has been imitated in several ways. Among the materials used for this are [[cubic zirconia]], synthetic [[spinel]], [[yttrium aluminium garnet]], and colored glass. A test of the stone with a [[dichroscope]] can easily distinguish these from genuine tanzanite, as only tanzanite will appear doubly refractive: The two viewing windows of the dichroscope will display different colors (one window blue, the other violet) when viewing genuine tanzanite, while the imitation stones are all singly refractive and will cause both windows to appear the same color (violet).<ref name=RGC>{{cite web |first=Antony |last=Zagoritis |title=Tanzanite synthetics and imitations |series=Gemology articles |website=The Rare Gemstone Company (theraregemstonecompany.com) |url=https://www.theraregemstonecompany.com/gemology-articles/tanzanite-synthetics-and-imitations }}</ref>

Synthetic [[forsterite]] ({{chem2|Mg2SiO4}}, the [[magnesium]]-rich [[Endmember|end-member]] of [[olivine]]) has also been sold as tanzanite, and presents a similar appearance. It can be distinguished from tanzanite in three ways:
# The first is by using a [[refractometer]]: Forsterite will show a [[refractive index]] of between 1.63–1.67, while tanzanite will show a higher index of 1.685–1.707&nbsp;.
# The second way is by using a special light filter: Genuine tanzanite will appear orange-pink, while forsterite will appear green when viewed through a [[Hanneman filter]].
# The third way is by examining a cut stone through its crown facets and viewing the [[brilliant (diamond cut)|pavilion cuts]] at the back of the stone using a standard jeweller's [[loupe]]: Forsterite will show [[birefringence]], making the pavilion cuts appear "doubled up", while the much lower birefringence of tanzanite will not have this characteristic.<ref name=RGC/>

Lower grades of tanzanite are occasionally enhanced by a [[vacuum deposition]] coating of [[cobalt]]; the cobalt layer imparts a deeper shade of blue. Similar to [[Cubic zirconia#mystic_coating|other gems']] [[optical coating|thin-film coatings]], the cobalt film does not adhere well to these stones: It tends to rub off over time, resulting in a much less intensely colored stone, although still tanzanite. The practice of cobalt coating is considered deceptive unless well-advertised.<ref name=RGC/>

==See also==
*[[Manyara Region]]
*[[Arusha Region]]

==References==
{{Reflist|30em}}

==External links==
{{Commons category|Tanzanite}}
{{Wiktionary}}
<!-- (Commenting this out because we generally don't keep lists of external articles like this, but if someone wants to use them to improve the article, or incorporate them as citations, that would be great.)
*[http://www.time.com/time/magazine/article/0,9171,1594137-1,00.html Time article of 2007 about the popularity of tanzanite].
*[http://www.time.com/time/magazine/article/0,9171,900582,00.html Article in the early stages of tanzanite (1969)]
*[https://web.archive.org/web/20080511210821/http://swalagemtraders.com/news/2008/02/06/4-tanzanite-july-7-1967-tanzanite-something-new-out-of-africa-but-no-one-knew-what-it-was Interesting article and documents from the early days: "Something new out of Africa but no one knew what it was"]
*[http://www.gemstone.org/gem-by-gem/english/tanzanite.html The ICA's tanzanite information page]
*[http://www.irinnews.org/Film/?id=4119 Gem Slaves – a short film from 2006 on tanzanite's child miners]
-->

{{Gemstone}}
{{Jewellery}}

[[Category:Epidote group]]
[[Category:Gemstones]]
[[Category:Minerals in space group 62]]
[[Category:Orthorhombic minerals]]