HomeMajor Colored StonesTanzaniteTanzanite, Zoisite Variety
Introduction and History

 

Part of the Zoisite mineral species and rare not only for its colour but also its scarcity, Tanzanite is named after the country where is it mined(Tanzania) and to date, has been found nowhere else on earth. And when the region that is producing these rare stones is mined out, there may not be any more tanzanite available.

 

Tanzanite, one of the most popular blue gemstones available today, occurs in a variety of shapes and sizes and also provides a striking assortment of tonal qualities. Rarely pure blue, Tanzanite almost always display its signature overtones of purple. In smaller sizes, Tanzanite tends toward the lighter tones, and the lavender colour is more common. While in larger sizes, Tanzanite typically displays deeper, richer colour.

Discovered in Tanzania in 1967. Maasai tribesman discovered Tanzanite in 1967, supposedly after lightening struck and caused a bush fire which heated the raw brown zoisite into the vivid blue-purple. Whether this is true or not is debatable. As a relative newcomer to the gemstone kingdom, and has only been on the market for about 30 years there will not be a lot of estate jewellery with tanzanite in case someone is making that claim.

 

How are Tanzanites priced?
This remarkably popular gem was discovered during the late 1960's in Tanzania's Great Rift Valley. A relative newcomer to the gem and jewelry scene, the last 30 years has seen tanzanite establish itself with great success becoming America's #1 selling gem after blue sapphire.

While arguably the easiest to assess of all the precious gems, understanding where value lies in tanzanite still requires some basic knowledge of this gem species. All the standard gem-value measurements are applied, but it must be remembered that prevalent tendencies of this richly coloured gem for large sizes and high clarities do affect their pricing structure.
The name tanzanite was a trade name coined by Tiffany & Co. shortly after the gem's discovery, an obvious allusion to its country of origin. This was thought necessary in order to make the stone marketable to the public: the name has since stuck as a varietal designation. Tanzanite's present-day popularity as a gemstone is largely thanks to Tiffany's marketing campaigns. The mining of tanzanite nets the Tanzanian government approximately USD $20 million annually, the finished gems later being sold mostly on the US market for sales totaling approximately USD $500 million annually.

In June of 2003, the Tanzanian government introduced legislation banning the export of unprocessed tanzanite to India (like many gemstones, most tanzanite is cut in Jaipur). The ban has been rationalized as an attempt to spur development of local processing facilities, thereby boosting the economy and recouping profits. This ban will be phased in over the next two years, until which time only stones over 0.5 grams are affected.

This is a serious situation for the city of Jaipur, as one-third of its annual gem exports are of tanzanite. Some members of the industry fear the ban will set a precedent, leading Tanzania to ban the export of all raw gem material, including the country's production of tsavorite, diamond and ruby. 

Colour Impacts Tanzanite's Value

Popularized by beautifully rich purple-blue colours, tanzanite frequently displays a deep intensity that is found nowhere else in the gemstone kingdom. Attractive eye-catching red flashes of fire (a "trichroic" light property unparalleled by other gems), often augments tanzanite's spectacular color. As with most other colored gemstones, the richer the color of the specimen, the more desirable it is.

Although traditionally thought of as an intensely purple-blue gemstone, tanzanite also occurs in a variety of other colors such as pink, green, ultramarine and a steely kind of blue. Frequently these tanzanites exhibit a color change from the more bluish hues under daylight, to pinkish violets under incandescent light.

Carat Weight of Tanzanite Impacts Per Carat Prices

Tanzanite frequently occurs in large crystal sizes and such specimens can be thought of as being fairly accessible. These large sizes also help to enhance tanzanite's perceived depth and richness of colour.

Similar to most other gems, when the carat weight of a Tanzanite increases, so does the price per carat. However, large tanzanite crystals with their abundant occurrence rates are simply more plentiful than ruby and sapphire. This means that weight related price jumps in Tanzanite are far less severe than in ruby and sapphire. Indeed, it is not uncommon for comparable quality 3 Carat, 4 Carat, and 6 Carat tanzanites all to have the same or very similar per carat prices - something that is unthinkable for ruby and sapphire.

Freedom From Inclusions

Large, gem quality clean tanzanite crystals are relatively common when compared to other precious gems. Well known for their predisposition for virtually flawless clarities, this tendency enhances sparkle, brilliance and ultimately beauty. While it would be incorrect to say tanzanite is a clean gem species, with so many near flawless tanzanites available on the market, obtaining such top quality clarities is within everyone's grasp - something that cannot be said of sapphire, sapphire or emerald.

Sources Of Tanzanite

The world's most predominant source of tanzanite is the Merelani deposit in Tanzania, East Africa. Located in the Rift Valley close by to Mount Kilimanjiro, the mines barely cover 20 square kilometers. With such limited deposits, it is likely that tanzanite can only become more valuable in the future.
Out of the individual deposits at Merelani, a subsection known as "D Block " has earned the reputation and kudos for producing the very best "AAA" quality tanzanites. While characterized by intensely deep purple colors with glistening flashes of red, tanzanite from the other locales such as "C Block", can, in reality produce gems as every bit as fine.

The Use Of Heat

Virtually all tanzanite is heated to about 500°C in order convert its unsightly brownish colouration to a more appealing blue. Most tanzanites seen on the market today have been subjected to heat treatment, an age-old practice thought to have originated in Sri Lanka some 2,000 years ago. Without this practice, virtually no tanzanite at all would be seen on the market today. Tanzanite is noted for its remarkably strong trichroism, appearing alternately sapphire blue, violet, and sage-green depending on crystal orientation. However, most tanzanite is subjected to artificial heat treatment to improve its colour: this significantly subdues its trichroism.

Natural Tanzanite is trichroic and usually has a brown/orange, purple, and blue axis.  Usually the brown/orangish coloured axis becomes intense purplish-blue when heat treated (almost all Tanzanite is heat-treated).  The blue and purple axis colours also may intensify greatly with heating. Untreated deep violet and blue crystals are possible but quite rare. Gems are usually cut on the deeper blue axis, but the purple axis also makes a terrific looking gem.

Tanzanite is relatively new to the coloured stone galaxy. This transparent blue gem first turned up in 1962, scattered on the Earth's surface in northern Tanzania, in eastern Africa. Scientists identified it as a variety of the mineral zoisite. About five years later, a prospector discovered a large deposit of it in the same area, and serious mining began.

Tiffany & Company recognized its potential as an international seller and made a deal to become its main distributor. Tiffany named the gem after the country it came from, and promoted it with a big publicity campaign in 1968. Almost overnight, tanzanite was popular with leading jewelry designers and other gem professionals, as well as with customers who had an eye for beautiful and unusual gems

Tanzanite's public recognition and popularity have grown steadily. But there have been wide fluctuations in the gem's supply and price level, due mostly to Tanzania's volatile political, social, and economic conditions. That country remains the gem's only source.

Tanzanites are heat-treated to produce colours that include light to dark violetish blue and bluish purple, as well as pure blue. Rich, deep hues are valued most, but you'll usually see these only in stones weighing 5 cts. or more. This is mainly because of decisions made during the cutting process. Tanzanite typically shows strong pleochroism, which means it displays different colours from different directions. It usually looks violetish blue from some directions, purplish from others.

Predominately blue tanzanite is generally worth more per carat, but because of the way tanzanite crystals grow, a cutter can usually get a bigger stone by orienting the gem to show the purple colour. With small rough, size is normally the main consideration. While the trade considers the pure blue stones to be the "top" grade, some customers actually prefer the lighter and more purplish colours.

Tanzanite is special-care gem for two reasons: sensitivity to thermal shock and the potential for cleavage. Sometimes the temperature change between the hot lights of the display case and the chilly glass countertop in an air-conditioned showroom can be enough to develop cleavages in tanzanite.

Because of its susceptibility to cleavage, tanzanite shouldn't be handled carelessly. Active people should consider jewelry that won't be exposed to as many bumps — pendants and earrings are good choices.

What's all this talk about Tanzanite "Blocks"?

Tanzanite, the delightful blue-purple member of the Zoisite family, has taken the world by storm in the last two decades. Sales of tanzanite have skyrocketed in recent years, exceeding sales of all other precious gems, except Sapphire. It is mined only in the East African area of Tanzania known as Merelani.

The tanzanite deposits are found in metamorphic rocks, marbles, and schists that belong to the Mozambique Belt (Rift Valley). The deposits run through the low hills of Merelani that rise from the Sanya plains near Mount Kilimanjiro. The deposit line periodically folds over itself creating concentrated pockets of tanzanite. These pockets provide gem miners with the richest pickings of these popular green, pink, ultramarine and purple-blue gemstones.

The tanzanite mining area has been divided into four sections called "blocks." These blocks are lettered A, B, C and D. The different blocks together total about 20 square kilometers and have been allotted to different mining concerns.

A gem quality tanzanite crystal. Typically found in concentrated folds along the mineshafts.
The Tanzanian Ministry of Mines allotted Blocks B and D to small-scale local miners to create local Tanzanian entrepreneurship. Together they employ approximately 10,000 miners across 750 shafts and mines. Block C was allotted to the big South African company, African Gem Resources (AFGEM) in 1999. Block A was allotted to Kilimanjiro Mines Ltd. and is currently not in service.

Block D has the reputation for producing the rare and beautiful "AAA" quality tanzanites. Characterized by intense purples with glistening flashes of red, the finest D Block ts can be likened to an old French wine of a rarest vintage. Many jewelers will never see a D Block tanzanite, as the vast majority of high quality tanzanite on the market comes from the neighboring C Block. tanzanite from the C Block can be AAA quality. But, the vast majority of tanzanite seen in jeweler's windows, while still gorgeous, is actually Block C tanzanite, not Block D.

As might be expected, the largest and most sophisticated techniques used in tanzanite mining take place in Block C. According to recent reports, there are three main shafts leading down from the surface—known as "Main", "Bravo" and "Delta.” The "Main" shaft, located in the middle of the Block. "Bravo" is situated towards the southern boundary of the Block, while "Delta" is located toward the northern boundary of the Block. An interceptor shaft, known the "JW", is cut into the "Main" block. Not only has "JW" produced the highest per ton yields (an amazing 60 Carats per ton) it will serve in the future as a ventilation shaft for the "Main," ensuring abundant airflow.

Past estimations have given the deposits a life of 20 years. Mining is currently taking place at about 170m in depth, with proven deposits of tanzanite bearing rock going down to about 280m. But if the new reports, which suggest tanzanite bearing rock goes down as far as 400m, is correct the life of the deposits will be extended considerably.

Chemistry and Crystallography
Chemical Composition Ca2Al3(SiO4)3(OH)
Crystallographic System Orthohombic
Cleavage Perfect
Fracture  
Fracture Lustre  
Habit  



Physical Characteristics

Hardness 6 - 7
Toughness  
Specific Gravity 3.35
Streak  



Optical Characteristics

Colour and Cause  
Degree of Transparency Transparent
Polish Lustre  
Refractive Index 1.692 - 1.700
Birefringence  
Optic Character  
Pleochroism Strongly trichroic
Dispersion  
Phenomena  
X-Ray Fluorescence  
Transparency to X-Rays  
Ultraviolet Fluorescence  
Chelsea Filter Reaction  
Absorption Spectra  



Description

Appearance  
Variety and Trade Names  
Misnomers  
Typical Size Range  
Typical Cutting Styles  

Inclusions 

Treatments
Unheated tanzanites typically show a greenish third trichroic colour.
Virtually every tanzanite is heat treated to permanently change its colour from orange-brown(much like the third trichroic colour) when it comes out of the ground to the spectacular violet-blue colour for which this precious gemstone variety is known. Heat treatment subdues much of the third trichroic colour, and may be used as an indication heat treatment. Heat treatment is assumed due to its prevalence.

Human heated tanzanite are not dichroic they are trichoic as all tanzanites. 

Trichroism is a property from the crystal system of Tanzanite which is orthorhombic. All stones from this crystal system are trichroic unheated or not. 

Now in some tanzanites while studying the dichoism in most unheated tanzanite the gamma ray can appear brown yellow to yellow green. This gamma ray give to many tanzanite a 
colour
 which is not vey appreciated. Using heat treatment this gamma ray turns blue to greenish blue turning the stone to a more attractive colour

But there are some unheated tanzanite for which the gamma ray is naturally green to blue meaning that these stones are not possible to seaprate from heated tanzanite using dichroism. 

The problem is not to separate unheated greenish tanzanite with heated blue, but unheated blue with heated blue of similar 
colour
s. 

To my knowledge the only way to separate without mistake heated and unheated tanzanite with a 100% reliability is to study the near UV speactrum of tanzanite in the 300 to 420nm range as Franck Notari from GIA research, Geneva (Gemtechlab) has shown. (See for reference his article in the "Revue Francaise de Gemmologie A.F.G.", n 141/142) 
It the case of unheated tanzanite the minumum absorption is in the 400nm range while for heated tanzanite it is in the 320nm range. 



Synthetics
None known to exist. But there exist an impressive tanzanite imitation in the form of synthetic forsterite with very strong pleochroism akin to tanzanite. 

Synthetic forsterite exhibit the venetian blind effect.

Origin 
Forms most often in granitic intrusions

Locality 
Mined only in Tanzania at the feet of the majestic Mount Kilimanjaro. Tanzania only commercially viable source

Care

Cleaning Methods  
Stability Avoid rough treatment. Sudden changes in temperature may cause cracking in tanzanite. The gem is stable to light, and can be attacked by hydrochloric and hydrofluoric acid.

Market 
Availability  
Public Recognition  
Wearability Good
Recommended Disclosures  

Key Separations, Suggestions for Testing and Evaluation

Strong Trichroism with two of the colours, blue and purple, being readily visible to the unaided eye. A Chelsea Filter will view the third reddish brown colour.

Possibilities for Confusion
Synthetic forsterite, Iolite

Bibliography and Suggested Further Reading
  • http://www.imperialjewels.com/info/tanzanite_prices.asp
  • http://www.gia.edu/library/4476/tanzanite.cfm

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