Tanzanite
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Tanzanite is the blue and violet variety of the mineral zoisite (a calcium aluminium hydroxyl sorosilicate), caused by small amounts of
vanadium Vanadium is a chemical element; it has Symbol (chemistry), symbol V and atomic number 23. It is a hard, silvery-grey, malleable transition metal. The elemental metal is rarely found in nature, but once isolated artificially, the formation of an ...
. Tanzanite belongs to the
epidote Epidote is a calcium aluminium iron sorosilicate mineral. Description Well developed crystals of epidote, Ca2Al2(Fe3+;Al)(SiO4)(Si2O7)O(OH), crystallizing in the monoclinic system, are of frequent occurrence: they are commonly prismatic in ha ...
mineral group. Tanzanite is only found in Simanjiro District of Manyara Region in
Tanzania Tanzania, officially the United Republic of Tanzania, is a country in East Africa within the African Great Lakes region. It is bordered by Uganda to the northwest; Kenya to the northeast; the Indian Ocean to the east; Mozambique and Malawi to t ...
, in a very small mining area approximately long and wide near the Mererani Hills. Tanzanite is noted for its remarkably strong trichroism, appearing alternately blue, violet and burgundy depending on
crystal A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macros ...
orientation. Tanzanite can also appear differently when viewed under different lighting conditions. The blues appear more evident when subjected to
fluorescent light A fluorescent lamp, or fluorescent tube, is a low-pressure mercury-vapor gas-discharge lamp that uses fluorescence to produce visible light. An electric current in the gas excites mercury vapor, to produce ultraviolet and make a phosphor ...
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. 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 A birthstone is a gemstone that represents a person's birth period, usually the month or zodiac sign. Birthstones are often worn as jewelry or a pendant necklace. History of birthstones Western custom The first-century historian Josephus bel ...
, the first change to their birthstone list since 1912.


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 Mount Kilimanjaro () is a dormant volcano in Tanzania. It is the highest mountain in Africa and the highest free-standing mountain above sea level in the world, at above sea level and above its plateau base. It is also the highest volcano i ...
. The mineral is located in a relatively complex geological environment. Deposits are typically found in the "hinge" of isoclinal folds.


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 Arusha Region () is one of Tanzania's 31 administrative Regions of Tanzania, regions and is located in the northeast of the country. The region's capital and largest city is the city of Arusha. The region is bordered by Kajiado County and Narok C ...
(presently Manyara Region). He was issued with a certificate of recognition three years later by the then President
Julius Nyerere Julius Kambarage Nyerere (; 13 April 1922 – 14 October 1999) was a Tanzanian politician, anti-colonial activist, and political theorist. He governed Tanganyika (1961–1964), Tanganyika as prime minister from 1961 to 1962 and then as presid ...
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.


Commercial history

In July 1967, Manuel de Souza, a Goan tailor and part-time gold prospector living in
Arusha Arusha is a city in Tanzania. The city is the Capital city, capital of the Arusha Region. It has a population of 617,631 people.
, found transparent fragments of blue and blue-purple gem crystals on a ridge near Mererani, some southeast of Arusha. He assumed that the mineral was
olivine The mineral olivine () is a magnesium iron Silicate minerals, silicate with the chemical formula . It is a type of Nesosilicates, nesosilicate or orthosilicate. The primary component of the Earth's upper mantle (Earth), upper mantle, it is a com ...
( 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 Nairobi is the Capital city, capital and largest city of Kenya. The city lies in the south-central part of Kenya, at an elevation of . The name is derived from the Maasai language, Maasai phrase , which translates to 'place of cool waters', a ...
-based consulting geologist and gemstone wholesaler who was then mining aquamarine in the region around
Mount Kenya Mount Kenya (Meru people, Meru: ''Kĩrĩmaara,'' Kikuyu people, Kikuyu: ''Kĩrĩnyaga'', Kamba language, Kamba: ''Ki nyaa'', Embu language, Embu: ''Kĩ nyaga'') is an extinct volcano in Kenya and the Highest mountain peaks of Africa, second-highe ...
. 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 Saks Fifth Avenue (Colloquialism, colloquially Saks) is an American Luxury goods, luxury department store chain founded in 1867 by Andrew Saks. The first store opened in the F Street and 7th Street shopping districts, F Street shopping distric ...
in New York. Hyman Saul took the samples to the
Gemological Institute of America The Gemological Institute of America (GIA) is a nonprofit institute based in Carlsbad, California. It is dedicated to research and education in the field of gemology and the jewelry arts. Founded in 1931, GIA's mission is to protect buyers and s ...
which correctly identified the new gem as a variety of the mineral zoisite. Correct identification was also made by mineralogists at
Harvard University Harvard University is a Private university, private Ivy League research university in Cambridge, Massachusetts, United States. Founded in 1636 and named for its first benefactor, the History of the Puritans in North America, Puritan clergyma ...
, the
British Museum The British Museum is a Museum, public museum dedicated to human history, art and culture located in the Bloomsbury area of London. Its permanent collection of eight million works is the largest in the world. It documents the story of human cu ...
and
Heidelberg University Heidelberg University, officially the Ruprecht Karl University of Heidelberg (; ), is a public research university in Heidelberg, Baden-Württemberg, Germany. Founded in 1386 on instruction of Pope Urban VI, Heidelberg is Germany's oldest unive ...
, but the first person to get the identification right was Ian McCloud, a Tanzanian government geologist based in Dodoma. Scientifically called "blue zoisite", the gemstone was renamed as ''tanzanite'' by Henry B. Platt, a great-grandson of
Louis Comfort Tiffany Louis Comfort Tiffany (February 18, 1848 – January 17, 1933) was an American artist and designer who worked in the decorative arts and is best known for his work in stained glass. He is associated with the art nouveauLander, David"The Buyable ...
and a vice president of Tiffany & Co., who wanted to capitalize on the rarity and single location of the gem and thought that "blue zoisite" (zoi′sīt) would not sell well. 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.


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. 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 Jaipur (; , ) is the List of state and union territory capitals in India, capital and the List of cities and towns in Rajasthan, largest city of the north-western States and union territories of India, Indian state of Rajasthan. , the city had ...
.) 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. In 2010, the government of Tanzania banned the export of rough stones weighing more than one gram. 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 , earning them $24 million. Following the construction of a perimeter wall around the mines, to improve security and prevent smuggling, production rose from in 2018 to a record in 2019. 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 Total reserves of tanzanite are estimated at , according to a report published in 2018. Block C, by far the largest site, has been estimated at with a Life of Mine (LOM) expected to last until the 2040s.


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, has introduced its own color-grading system. 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 Anisotropy () is the structural property of non-uniformity in different directions, as opposed to isotropy. An anisotropic object or pattern has properties that differ according to direction of measurement. For example, many materials exhibit ver ...
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
dichroic In optics, a dichroic material is either one which causes visible light to be split up into distinct beams of different wavelengths (colours) (not to be confused with dispersion), or one in which light rays having different polarizations are ab ...
. The dichroic colours range from violet through bluish-violet to indigo and violetish-blue to blue. 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). 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

Tanzanite forms as a brownish crystal and is trichroic, which means it shows three colours – brown, blue and violet – concurrently. Heating, either underground naturally by metamorphic processes, or artificially, removes the brown or burgundy colour component to produce a stronger violet-blue color and makes the stone "dichroic", which means it only reflects blue and violet. 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. Heat-treating in a furnace is usually carried out at between 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. 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 wildfire 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 The Gemological Institute of America (GIA) is a nonprofit institute based in Carlsbad, California. It is dedicated to research and education in the field of gemology and the jewelry arts. Founded in 1931, GIA's mission is to protect buyers and s ...
states that the source of heating is gemologically undetectable, but is assumed because of its prevalence. Tanzanite may be subjected to other forms of treatment as well. Recently, coated tanzanites were discovered and tested by the AGTA and AGL laboratories. A thin layer containing
cobalt Cobalt is a chemical element; it has Symbol (chemistry), symbol Co and atomic number 27. As with nickel, cobalt is found in the Earth's crust only in a chemically combined form, save for small deposits found in alloys of natural meteoric iron. ...
, determined by
X-ray fluorescence X-ray fluorescence (XRF) is the emission of characteristic "secondary" (or fluorescent) X-rays from a material that has been excited by being bombarded with high-energy X-rays or gamma rays. The phenomenon is widely used for elemental analysis ...
, 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

Pleochroism Pleochroism is an optical phenomenon in which a substance has different colors when observed at different angles, especially with Polarization (waves), polarized light. Etymology The roots of the word are from Greek (). It was first made compou ...
has a physical property in which the gemstone will appear to have multiple colours based on the angle of the light hitting the stone. Tanzanite is a pleochroic gemstone. Most Tanzanite are blue when viewed from one direction but can vary from violet to red when seeing 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.


Imitation and cobalt-coated tanzanite

, 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 Cubic zirconia (CZ) is the cubic crystalline form of zirconium dioxide (ZrO2). The synthesized material is hard and usually colorless, but may be made in a variety of different colors. It should not be confused with zircon, which is a zirc ...
, synthetic
spinel Spinel () is the magnesium/aluminium member of the larger spinel group of minerals. It has the formula in the cubic crystal system. Its name comes from the Latin word , a diminutive form of ''spine,'' in reference to its pointed crystals. Prop ...
,
yttrium aluminium garnet Yttrium aluminium garnet (YAG, Yttrium, Y3Aluminium, Al5Oxygen, O12) is a synthetic crystalline material of the garnet group. It is a Crystal system, cubic yttrium aluminium oxide phase, with other examples being YAlO3 (YAP) in a Crystal system, ...
, and colored glass. A test of the stone with a
dichroscope A dichroscope is a pocket instrument used in the field of gemology, and can be used to test transparent gemstones (crystals). Experienced gemologists, observing the pleochroism of some gems, can successfully detect gemstones from other artificial ...
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). Synthetic
forsterite Forsterite (Mg2SiO4; commonly abbreviated as Fo; also known as white olivine) is the magnesium-rich Endmember, end-member of the olivine solid solution series. It is Isomorphism (crystallography), isomorphous with the iron-rich end-member, fayalit ...
(, the
magnesium Magnesium is a chemical element; it has Symbol (chemistry), symbol Mg and atomic number 12. It is a shiny gray metal having a low density, low melting point and high chemical reactivity. Like the other alkaline earth metals (group 2 ...
-rich end-member of
olivine The mineral olivine () is a magnesium iron Silicate minerals, silicate with the chemical formula . It is a type of Nesosilicates, nesosilicate or orthosilicate. The primary component of the Earth's upper mantle (Earth), upper mantle, it is a com ...
) 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 A refractometer is a laboratory or field device for the measurement of an Refractive index, index of refraction (refractometry). The index of refraction is calculated from the observed refraction angle using Snell's law. For mixtures, the index ...
: forsterite will show a
refractive index In optics, the refractive index (or refraction index) of an optical medium is the ratio of the apparent speed of light in the air or vacuum to the speed in the medium. The refractive index determines how much the path of light is bent, or refrac ...
of between 1.63 and 1.67, while tanzanite will show a higher index of 1.685 to 1.707. The second way is by using a Hanneman filter: through it, genuine tanzanite will appear orange-pink, while forsterite will appear green. The third way is by examining a cut stone through its crown facets and viewing the pavilion cuts at the back of the stone using a standard jeweller's loupe: forsterite will show
birefringence Birefringence, also called double refraction, is the optical property of a material having a refractive index that depends on the polarization and propagation direction of light. These optically anisotropic materials are described as birefrin ...
, making the pavilion cuts appear "doubled up", while the much lower birefringence of tanzanite will not have this characteristic. Lower grades of tanzanite are occasionally enhanced using a layer of
cobalt Cobalt is a chemical element; it has Symbol (chemistry), symbol Co and atomic number 27. As with nickel, cobalt is found in the Earth's crust only in a chemically combined form, save for small deposits found in alloys of natural meteoric iron. ...
, as cobalt imparts a deeper shade of blue. The cobalt layer does not adhere well to these stones, and tends to rub off over time, resulting in a much less intensely colored stone. Though still tanzanite, the practice of cobalt coating is considered deceptive unless well-advertised.


See also

* Manyara Region *
Arusha Region Arusha Region () is one of Tanzania's 31 administrative Regions of Tanzania, regions and is located in the northeast of the country. The region's capital and largest city is the city of Arusha. The region is bordered by Kajiado County and Narok C ...


References


External links

{{Jewellery Epidote group Gemstones Minerals in space group 62 Orthorhombic minerals