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Heated or unheated gemstones?

by Julie MIALET 24 Sep 2022

Some of you will ask yourself this question when buying a precious or fine gemstone.

Man has always heated stones to increase the quality of colour or transparency of the stone. In fact, heat treatment causes alterations in the physical and chemical structure of the gem and improves the colour or purity of the gemstones.

Traditional "blow pipe" heating in Thailand, copyright GIT

It is a stable and absolutely tolerated process on the gemstone market. It is part of the customs of the profession.

Thus the heat treatment of corundum (rubies and sapphires) has been practised for several hundred years now.

Currently, almost all rubies and the majority of sapphires produced on the market are heat treated. The heat treatment of corundum has been used for centuries and is therefore considered a common practice and even a traditional use.

Rutile needles in unheated sapphire, Copyright withclarity

In recent decades, following the invasion of “heated stones” on the market (some 80% to 95% of corundum), some purists have been looking for stones that have not been thermally modified by human hands.

This is what is more commonly known as “unheated” stones.

An unheated ruby or sapphire, with a certificate of authenticity, will see its price slitghly increase when the colour is beautiful and the stone important.

Inclusions in an unheated Ceylon yellow sapphire.

On the other hand, it is not because a sapphire or ruby is “heated” that it should not be of interest.

There are magnificent heat-treated stones, whose colour has nothing to envy an “unheated” stone.

Don’t forget that you should buy a stone because of the emotion it gives you…and your budget!

If possible, ask for a laboratory certificate: Laboratoire Français de Gemmologie, Gem Paris, Carat Gem Lab are recognised laboratories in Paris.

"silks" inclusions in an unheated ruby from Mozambique

The heat treatment does not need to be mentioned on the certificate, but some recent treatments do.

These so-called specific treatments, by diffusion, bombardment, coloured oiling, filling with resins, are the result of new processing techniques.

Concerning emeralds, these stones cannot be heated due to their great fragility, the profession tolerates what we call “oiling”.

Oiling is intended to fill cavities or cracks opened on the surface by inclusions, thanks to a colourless oil.

If you are interested in gemstones, don’t hesitate to discover what les Pierres de Julie have to offer in the store…

We are also available for any free expertise of your jewelry, for that you can send an email with your photos to

Whether you're interested in buying a piece of jewellery or a stone, a free jewelry appraisal or jewelry resale, Pierres de Julie welcomes you to its Parisian boutique in the Village Suisse, a stone's throw from the Ecole Militaire and the Esplanade des Invalides.

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