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Heated vs. Unheated Sapphires, what's the difference?

At Skyjems, we often get asked what the difference is between heated and unheated sapphires. Before we talk about how this beautiful gemstone can be heat treated (and why), let’s get into some general details about sapphires first.

Sapphires are a variety of the mineral corundum and come in every colour imaginable, though it’s the finest
blue stones that are most coveted by jewellers and collectors worldwide. The versatile gemstone has remained popular throughout history and continues to be set in a variety of jewellery styles ranging from radiant engagement rings to beautiful pendants as a timeless expression of elegance and beauty.

Heat processes can occur naturally to produce a superior gemstone, however many of today’s sapphires are often heat treated to improve or enhance their colour. These types of heating processes have been carried out for centuries, though several different techniques have been identified in more recent times. In cases where a sapphire is heat treated, this information should always be disclosed to the buyer.

Today’s sapphires are typically heat treated, and there are currently three main heating processes currently used that actually mimic natural processes:

  1. Heating to remove silk and/or asterism from the sapphire
  2. Heating without oxygen to darken a sapphire (reductive heating)
  3. Heating with oxygen to lighten a sapphire (oxidative heating)

Sapphires can be subjected to one or more of the above heat treatments to enhance their colour depending on the chemical composition of the individual gemstone. For example, an overly dark sapphire can be heated for a day or so at temperatures between 1,000 and 1,600 degrees Celsius to oxidize and reduce the amount of ferric iron present and thus lighten the gemstone’s hue.

The heat treatment process improves gems with zoning (or inconsistent colour) that might otherwise be considered unattractive, so that they can be used as a gemstone for a beautiful engagement ring or other jewellery piece.

But are there any downsides to a heated sapphire?

For the treatments outlined above, the simple answer is no. The gem will have the same stability and durability as an unheated sapphire, and the price point will actually tend to be lower for a heat treated sapphire than for a natural, unheated gem.

On the other hand, there are some forms of heat treatment that cannot be said to be as durable and are therefore not as popular. For example, the now-common practice of diffusion treatment for sapphires affects the gem value tremendously and should always be disclosed to the buyer.

Additionally, diffusion treatments are typically simply a surface treatment for the gemstone, meaning that only a few tenths of a millimetre are actually coloured blue in a blue sapphire, for example. If you compare it to putting on nail polish, the layer of polish over your nails would actually be thicker than the coloured surface of a diffusion treated sapphire. Like nail polish, the sapphire’s surface can chip or become abraded fairly easily, revealing blemishes in the colour that might include flashes of yellow, green, or brown beneath the initial blue layer.

If a high-quality blue sapphire gets chipped or scratched, it will maintain a consistent blue hue and be relatively easy to repair. A diffusion treated gem, on the other hand, would require retreatment after being recut.

Diffusion treated sapphires are a good stand-in for the more expensive untreated or heat-only treated gems. However, as mentioned before, their price and durability are drastically different from heat-only treated or untreated gems, so a concern with these diffusion treated stones can actually be unethical dealers!

Unfortunately these less durable gemstones are often sold by some retailers as genuine heated sapphire. They may not
technically be lying, but they might choose to leave out critical information which could result in you spending far more in the long run than is justified for these gems.

Your dealer choice is crucial for this reason. Honest gem dealers will always disclose treatments to you and give you a money-back guarantee on their stock. offers many certified sapphires and provides a 100% guarantee on our stock, and we provide Gemological Institute of America (GIA) certificates for many of our high-end gems. 

This should be expected from any dealer in this field. If someone tries to sell you a gemstone or piece of jewellery that does not have these guarantees, caveat emptor: buyer beware.

Heat treated sapphire is here to stay, though natural, unheated sapphires are the most ideal gems to choose whenever possible to help ensure that your purchase incurs a greater increase in value over time.

If you are looking for heat treated sapphires, please make sure that your dealer is reputable, offers a guarantee on the gem, and provides a laboratory certificate from a major gem lab such as GIA, The International Gemological Institute (IGI), or GemResearch Swisslab.

Taking these precautions when choosing your gem dealer will ensure that you get a fine sapphire gemstone that will last your whole lifetime and beyond.

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