How to judge Tanzanite

RARER than diamond, emerald, ruby and sapphire, Tanzanite has become one of the must-have coloured stones for discerning women, symbolising a combination of Love, Luxury, Fashion, Faith and Security. Discovered in 1967 in Merelani Hills of Tanzania, Mount Kilimanjaro, the blue-violet crystal not only became an instant curiosity but the strong pleochroic gemstone, displaying distinct colour variation along different axes, was something unusual. When observed through a dichroscope, one could see a sapphire blue, a distinct purple and light green; in another direction, a light blue and near colourless shade; and a purple, a light blue, and a green.

Colour me Tanzanite: Judging quality The body colour of tanzanite is variable from strong blue violet to a highly saturated sapphire blue. The richer the colour the more valuable the stone is. Some people prefer an appealing violet while others find a rich blue more desirable. Generally the demand is for stones showing the least violet. Tanzanite also occurs in brown, pink, greenish and yellow. All these colours may turn to blue after heat treatment. The results of this treatment are stable. Without heat treatment, tanzanite would be unmarketable. The colour of tanzanite is most intense in large stones above 10cts. Sometimes referred to as green tanzanite or chrome tanzanite, a pale to dark green, yellowish green, bluish green to greenish blue variety has begun to appear in the marketTo judge colour, lighting is important. Under incandescent light tanzanite tends to shift towards violet. The best stones show a vibrant blue under any light.

Clarity and cut: Generally stones with visible inclusions are not desirable. Most stones are eye clean. This has to do with heat treatment. The size and shape of the rough determines the final quality of cut. Transparent material is faceted and chatoyant stones are cut cabochon. Ovals and cushions are the most common cut, but other cuts such as rounds, emerald cuts and fancy cuts are available in the gem market. The most popular sizes for jewellery use are in the 15-20cts range. Tanzanite can display the gemstonality of superb blue sapphires for a fraction of the price. Prices tend to fluctuate periodically due to irregular production at the source, but fine quality stones in the 40-50cts range usually fetch less than USD1,000 per carat at retail.

Synthetics and simulants: Tanzanite has never been synthesised on a commercial scale but the most commonly found simulant is blue violet glass. A synthetic fosterite has also been used as an imitation.

Handling: Never clean tanzanite with ultrasonic cleaners and the stones should never be exposed to hydrochloric and hydrofluoric acids. They will destroy the stone. Steamer is also not safe. The best way is to clean the stone with warm soapy water.

As a gemstone, tanzanite has its merits and defects. In terms of durability it is a problem stone. Due to its low hardness (6.5) and easy cleavage, it can wear quickly and damage easily when exposed to mechanical shock and careless application of heat during jewellery repair. Jewellery repairmen with no gemological background have frequently destroyed stones because they couldn’t recognise tanzanite from sapphire.

Despite these problems tanzanite is an attractive stone and at times you wish that sapphire could reach its beauty.The stone has lately been chosen as December’s new birthstone - a unique Christmas gift for women of all ages.