Be wary of retailers touting “unheated” tanzanite for a price premium. All tanzanite has to be heated in order to have its distinctive blue color. A very small percentage of tanzanite is excavated already blue; the result of natural heating underground. The rest is heated artificially after it is mined, to remove any brown in the stone – which may be veiling the blue color.
Tanzanite which has not been heated after excavation is very rare. It is also a common misconception that extended duration of heating makes tanzanite bluer. The blue color is actually determined by the amount of vanadium that was present when the stone formed, and the heating process is not capable of changing that; it only removes any brownish color in the stone to better reveal the blue.
Many people find the idea of extra rare tanzanite very appealing and may be enticed by promises of “unheated” tanzanite. However, before you pay more for the rare piece of tanzanite on offer, you should be aware that this claim can’t be substantiated: there is currently no scientific way of telling if tanzanite has been heated before or after mining.
* It is important to note that tanzanite is “heated” – not “heat treated”. The latter term is used by gemologists to describe sapphires that have been heated for days at temperatures close to melting point, which changes their chemical structure and dramatically affects their value.