Named by Tiffany & Co after Tanzania, the country in which it was discovered, Tanzanite has become one of the most popular gemstones on the market, and may fetch enormous prices due to its high demand and flattering colours.
Some sources have predicted that the current availability of Tanzanite is quiet minimal, and all mines may be obsolete within the next 5-10 years. This has caused a flurry of purchasing, with consumers keen to add this beautiful gem to their collections.
Tanzanite is the blue/violet variety of the mineral zoisite belonging to the epidote group. It was discovered in the Mererani Hills of Manyara region in Northern Tanzania in 1967, near the city of Arusha and Mount Kilimanjaro.
In 2002, the American Gem Trade Association chose Tanzanite as a December birthstone, the first change to their birthstone list since 1912.
Tanzanite forms as a brownish crystal and is trichroic - which means it shows three colors - brown, blue and violet concurrently. Heating - either occurring underground naturally by metamorphic occurrences or above ground by man in a furnace removes the brown or burgundy color component to produce a stronger purplish-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, most 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.
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" stone 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. The GIA 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, 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.