Did you know that many of the sapphires and rubies on the market today have been heat treated to enhance their colour and clarity?

The practice is now so common that you will often see sapphire or ruby specifically labelled as “unheated” since these stones attract a higher value. Certificates may also make statements such as “no evidence of heat treatment”.

Demands of the market

It is thought that we have been heating sapphires to improve their quality for many centuries, quite possibly from Roman times. However, since the 1980s, the high demand for top-quality corundum (sapphire and ruby) has meant that many gem dealers are using this process on a much wider scale than ever before.

The heating process

The use of “simple” heat treatments (i.e. a one-off treatment rather than a more lengthy process) has become generally acceptable within the marketplace. Many gemologists think that such a treatment is simply an extension of the natural process of forming a gemstone which was, after all, created at a high temperature within the Earth’s crust.

The temperatures for this process can range from 500°C up to 1800°C (the melting point for corundum is just over 2000°C) and the results can vary considerably, depending on factors such as the impurities in the original material, the method used and the standard of the heating equipment.

The end result

Once completed, the changes caused by the heating process are stable, permanent and irreversible. Also, the process does not weaken the stone in any way.

After a stone has been heated, many of the natural inclusions (flaws) are altered or removed. This means that identifying the country of origin can be very difficult (sometimes impossible), even for a highly-skilled gemologist. Some inclusions will, however, allow you to identify a heat-treated gemstone. A key feature is the presence of included crystals surrounded by “halo-like” fractures. Also, small, white “pinpoint” inclusions are a sign of heat treatment.

Advice for buyers

All reputable gem dealers should be willing and able to inform their customers if a stone has been heat treated. At Nineteen48, we operate a policy of full disclosure on all our stones.

Buyers should note that it is less likely a stone with a pale colour has been subjected to heat treatment (although heating under certain conditions can be used to lighten the colour of very dark sapphires). In the general market, it is often safe to assume that a sapphire or ruby with a deep, uniform colour and a high clarity has been treated in some way.

So when you are shopping for a gemstone, please make sure that you at least ask the question of the dealer regarding heat treatment and ensure that you make an informed buying decision.