I’m trying to find a test that will determine whether or not a tanzanite is unheated
There is really no way to tell if Tanzanite has or has not been heated and those touting that it has not are usually commanding higher prices. However the industry is fully aware that most Tanzanite has been heated, slightly or otherwise and it does not have an effect on the pricing of the material. Most contend that it’s just what nature forgot to do on these types of gem materials. Keep in mind that Tanzanite could have been heated by nature or by a laboratory and there is no known test on the market that even satisfies that a lab vs nature did this. Again, the market is well aware that most Tanzanites have been heated and again, it has little to no effect on the pricing. As with most gems today, it’s just common. As noted by the GIA, Untreated, tanzanite is typically brownish as the heat treatments are what generally give it the blue colors.
As a gem dealer and collector would I think it more rare or even want to pay for a gem that has not been heated, particularly tanzanite, than one that has been heated, either naturally or by laboratory means. No. To me it is more of scientific question really. If most unheated Tanzanite’s are say, according to the GIA, typically brownish then can we really even say then that the gemstone is a Tanzanite as most consider Tanzanite to be blues, and purples? It’s much like saying, is it a Pink Sapphire or Ruby, and as the old saying goes, it depends on if you’re the buyer or the seller. But as noted, I would not pay premium just for a brown piece of rock.
I have found in my experience that heated Tanzanite ( lab not natural heat ) can be dominant in one or two colours eg: blue/violet whereas Tanzanite heated whilst in the ground naturally displays many colours blues/ violets/ purples/ lilacs/ Reds / pinks even greens - exhibiting true pleochroism -
Also: what about a Green, Pink, Yellow, Peach Tanzanite- or even Chameleon Tanzanites ? All reputed to be Natural - not heated/treated and also reputed to be even rarer than Investment grade blue/violet, violet/ blue Tanzanites.
as you would be aware Tanzanite is Trichroic, and displays 3 different colours depending on which orientation of the rough you are looking at. yes it can look brownish but also purplish blue brown and violet
I have one 10 carat piece of rough that it trichroic in colour not heated in anyway.