A relatively recent addition to the gemstone market, tanzanite’s limited origin and brilliant blue color has helped it climb the jewel market ladder with great speed. A stone so much rarer than even diamonds, tanzanite in the past decade has owned the position of the second most loved colored gem in the United States, preceded only by sapphire.
What few people are fully aware of, is the fact that when rough tanzanite is mined from the foothill of Mt. Kilimanjaro, more often than not it doesn’t look at all as it does in the market. Instead freshly mined rough tanzanite loose stones are a dull yellow-brown in color, and it takes skill to even differentiate it from the base rock at times.
If you’ve been scouting the market to buy tanzanite stones, you should have a decent idea of the treatments it goes through. Loose tanzanite gem stones are the hardest to sell as they have no embellishments masking its flaws, so often jewelers tend to overcompensate for these by excessively treating the rough stone – increasing its outer beauty but degrading the value and quality.
Once mined, rough tanzanite stones are heated to approximately 500 degrees (sometimes even more depending on the quality of the stone) in crucibles. This treatment causes changes in the crystal, and once the treatment is complete and the gem is taken out and cooled, its changed properties are retained permanently. Depending on the degree of heat it bears, its color intensifies – changing it from the dull brown to a permanent blue (ranging between pale sky blue, rich cobalt and violet).
This treatment can also be done once the rough stone is faceted – that is once it has been cut to the desired shape and its outer planes have been angled ideally to maximize refraction (hence its sparkle).
Treatments impact the color to a great degree, but not all tanzanite gems are blue. Heat treatment doesn’t change the color of the rough stone, it only brings out and intensifies the already existing shade. Hues range from blues to violets to red-purple, some even with tints of grey, brown and green. Even so, the most valuable still remains the deep blue ones.
Tanzanite is originally a form of zoisite – chemical composition and all. Its name is specific to this crystalline form, which means that its properties are the same as that of zoisite – originally a rock that is brown, pink or grey in color.
Of course, don’t be fooled, tanzanite loose stones do come out of the ground in their original blue shade as well; having been naturally heated inside the earth by the temperatures of Mt. Kilimanjaro – a volcanic mass in itself. It just so happens that even though naturally blue tanzanite is most valuable, treated tanzanite has been accepted by the market as the next best option.
Lastly, while heat treatment is accepted, there are other measures taken by jewelers and distributers to enhance the color and shine of the tanzanite. When heated, tanzanite crystals tend to ‘crackle’, which is every miner’s nightmare as it loses its value. Hence they use other techniques – such as coating the gem with cobalt and the like. If you’re shopping for loose stones, keep a check on quality and make sure you get a certificate of authenticity.