Tanzanite

The mineral zoisite had long been known when a beautiful, transparent blue stone was discovered in Tanzania in 1967. The then-president of Tiffany & Co described it as being the most beautiful blue stone to be discovered for 2000 years. At first the gemstone was called by its mineral name, zoisite, but once it became obvious that this sounds like the word “suicide”, the trading name was changed to Tanzanite, after its country of origin. The stone has a hardness of 6.5 to 7, making it relatively soft, and it is naturally blue, violet and burgundy. When fired at a temperature of around 320 to 500 degrees Celcius, it develops its famous blue colour. Virtually all tanzanite on the market has been heat treated and this is internationally accepted. Given that tanzanite changes its colour at a relatively low temperature, goldsmiths must take great care when applying heat to jewellery containing tanzanite to avoid any negative effects on the colour of the gemstone.Tanzanite also shows a strong tendency towards pleochroism, meaning that when viewed from different angles the stone has different colours (e.g. blue and grey) or at least different shades of blue. This is a natural phenomenon, but for investment purposes it is undesirable, as high quality gemstones are expected to have a uniform colour.Legend has it that some workers threw a couple of blue-brown stones onto their camp fire after their day’s work. When they got up the next morning, they found scintillating blue stones in the ashes of the fire. It is doubtful that there is any truth in this tale; far more likely is that experiments were conducted on the discovered material, but it does make for a good campfire story…As heat can eliminate unwanted burgundy and violet hues, nearly all tanzanite on the market today has been heat treated. Strongly blue natural stones only reach the market very rarely; here one can expect a price of around 4000 Euro per carat for large stones of 10 ct or more. It is notable that tanzanite also occurs in large sizes; cut examples weighing over 100 ct are available on the market.The 1997 cinematic film “Titanic” contained a subtle marketing measure: in the film the leading lady throws a necklace with a large blue stone overboard. Due to its size and vibrant colour, it is clear that the stone in question is not a sapphire, but rather a tanzanite. This scene gave tanzanite, which was at the time suffering a price fall, a huge boost in popularity among the public, which led to higher demand and thus a very strong increase in value that lasted for several years.As far as investments in tanzanite are concerned, there are currently two opinions: the area in Tanzania where the stone occurs is only 7 km long and 1.2 km wide and supplies will soon be exhausted. Large, intensely gleaming and visually pure tanzanite gemstones are becoming rarer and rarer.On the other hand, no other gemstone has been subject to the whims of fashion to the extent of tanzanite. Extensive marketing by Tiffany’s and the film “Titanic” have led to huge movements in the price of tanzanite; at times minus 50% followed by an increase of 100%. Currently the price is around 10 – 20 % above the minimum price, meaning those who find the stone desirable and trust in the supply source being exhausted in the near future can purchase at a relatively low cost.Purchased stones should definitely be fairly large (5 – 15 ct or more) with a deep blue colour. We advise against the purchase of paler, small tanzanites. In contrast to other gemstones, tanzanite can be purchased for a relatively low price per carat.Certificates only play an important role for untreated tanzanite; a certification of origin is unnecessary, as the stone has only ever been discovered in the aforementioned small area of Tanzania. Be careful with the idea that it might be cheaper to buy tanzanite directly near to where it is mined in Africa. The price there is usually three to four times the European price.