All about Tanzanite heat treatment – does it change the price ?

Freshly mined tanzanite crystals.  Some are already blue and some are brown. The brown will be heated – but afterwards if you were to mix them all up, even a scientist cannot tell if they were mined blue or if the had to be heated artificially to remove the brown. This is because ALL tanzanite forms brown , but some crystals change color underground, ” naturally” by themselves – as the result of metamorphic heat or geological events. Thus , all tanzanite is assumed to have been heated in someway and there is no legit price premium associated for so called  “unheated” tanzanite.

Tanzanite typically forms as a brownish mineral and must be heated to reveal its distinctive blue color. However, it is impossible to determine whether this heating occurred naturally underground or in a controlled environment after mining.

Natural vs. Artificial Heating
Heating underground can occur naturally due to geological pressures and metamorphic conditions. Alternatively, after mining, brownish tanzanite is often gently heated in an oven for approximately 30 minutes at temperatures not much higher than those used for baking a cake.

Industry Standard: All Tanzanite is Heated
As a standard practice, the gemstone industry assumes all tanzanite has been heated. This assumption has no effect on its value. Furthermore, until recently, there was no scientific method to conclusively prove whether the heating occurred naturally underground or artificially above ground. For this reason, no reputable laboratory, including the GIA (Gemological Institute of America), issues certificates explicitly stating whether a tanzanite is heated or unheated. Most certificates simply state “no evidence of treatment” or “cannot be determined.”

Recent Scientific Developments
Over the past few years, some scientific advancements suggest it might be possible to identify the origin of heating. However, these findings have not been adopted into the certification process by major gemological laboratories like the GIA.

Heated but Not Heat Treated
Tanzanite is not “heat treated” in the same way as many other gemstones, such as sapphires or rubies.  Usually, heat treatment in gemmology involves exposing gemstones to extremely high temperatures—sometimes close to their melting points—for weeks or months. This process can significantly alter the stone’s chemical structure and, in turn, its value. For instance, the price difference between a heat-treated and unheated sapphire can amount to thousands of dollars per carat. However, this type of treatment does not apply to tanzanite.

Is Tanzanite Direct Tanzanite Heated?
While geologically, it is impossible to be 100% certain if a tanzanite has been heated after mining, on this website I always state that it is, to ensure complete transparency with you, my clients.

Does Heated Tanzanite Sell for a Different Price?
No, heated tanzanite does not sell for a different price. Since it is currently impossible to determine whether the heating occurred naturally underground or artificially after mining, the industry does not differentiate between the two. As a result, there is no legit price premium associated for  “unheated” tanzanite.

What is More Valuable?
All tanzanite is assumed to be heated, whether by geological processes or through post-mining enhancement. This assumption has no impact on value. Unlike gemstones such as sapphires or rubies, where unheated specimens command significantly higher prices, heating does not affect the valuation of tanzanite.

A Note of Caution
Be wary of retailers claiming to sell “unheated tanzanite” at a premium price. Since reputable gemological laboratories like the GIA do not issue certificates verifying whether tanzanite is unheated, such claims are typically unfounded and misleading.

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