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Heated Vs Unheated Tanzanite; How to Tell Tanzanite Quality

December 26, 2018 · 4 min read

Natural Tanzanite gemstone is a mix of grey, green, brown and blue colours. The gem achieves its blue colour through heat-treatment. Most of the Tanzanites we see in the market are heated. The big question is does heating affect Tanzanite quality?

Heating can occur naturally before the gem is mined. The earth’s thermal vents have been heating Tanzanite deposits for millions of years hence some gems are mined being already blue in colour. After mining, the gems are usually heat treated regardless, in order to remove any residual unwanted colours.

Natural Tanzanite gemstones can also be heat-treated to remove the brownish colour and leave behind the blue and violet that we have all come to adore. Heating is done in temperatures ranging from 400 to 600 degrees centigrade for about an hour.

The heat treatment of Tanzanite is very mild as compared to heat treatment of other gems like rubies and sapphires. Such gems can be heated to temperatures between 1000 and 1800 degrees centigrade and held at those temperatures for days or weeks.

Tanzanite’s colour change through heating is not reversible and will remain unaffected by cutting and polishing. Be warned the colour does not signify Tanzanite quality.

Is Heated Tanzanite more Valuable than Unheated Tanzanite?

While heat treatment of Tanzanite may result in a more favourable colour, it does not enhance the grade or value of the gemstone. If a piece of Tanzanite was medium-grade, heating will not enhance it to a top-grade Tanzanite. Heat treatment does not affect Tanzanite quality. 

The value of Tanzanite will still depend on other factors apart from colour. These are the 4 Cs of Tanzanite. Along with colour, there are Carat, Cut and Clarity factors that determine the value of Tanzanite

Will Heating Enhance Tanzanite’s Color Saturation?

Heating does not change the original colour of this rare blue gemstone. It only removes the red/brown axis and leaves behind the desired blue/violet colour. If the stone was originally pale blue, heating will not enhance it into a deeper blue.

When a medium-grade Tanzanite which is normally lightish brown is heated, it will become pale blue/violet. When a top-grade stone which is normally deep brown is heated, it will become a deep blue/violet after heating. 

Inspecting Tanzanite quality; Rare blue tanzanite stones

How to Distinguish Heat-Treated vs Unheated Tanzanite?

The difference between heated and unheated Tanzanite is the removal of the yellow colour due to heating. However, it is known in the mining industry that some Tanzanite has been naturally heated and may show no yellow colour when extracted.

Technically speaking, all Tanzanites are heated. Either through pre-mining thermal vent heating or post-mining in commercial jewellers. Heat treatment is a common practice and one should expect this when buying Tanzanite.

The practice goes back centuries. Heat treatment is basically a continuation of the heating processes that occurred in the ground. In today’s heat treatment, the stones are heated at very high temperatures hence speeding up the heating process that would have taken years if the stones were to be left in the ground.

Only a very small amount of untreated Tanzanite with perfect colour is found in the mine. This type of naturally blue Tanzanite has a much greater demand in the market as they are rare.

Without heat treatment, the world would be with less coloured gemstones, Tanzanite included. Heating allows us to change off-coloured stones into beautiful coloured ones.

Does Heat Treatment Affect Tanzanite Quality?

It is currently impossible to scientifically test the nature of the Tanzanite’s treatment, whether naturally-heated or man-heated. Since all Tanzanite has been heated in one way or another, there is no effect on price. It is a common assumption that all Tanzanite gems have been heated at some point.

With that said, we offer in our online shop a variety of premium natural Tanzanite jewellery for you to buy at your convenience. We’re reputable Tanzanite dealers that mine, produce and sell only the best quality products to our clients.

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