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Unheated Sapphires versus Heated Sapphires

Updated: Mar 3

The question I get asked time and time again by my customers is, "what is the difference between a heated sapphire and an unheated Sapphire"? This educational blog will cover everything you need to know about this process and what it means for Sapphires (and Ruby), which both belong to the Corundum family.

98% of Sapphires on the worldwide market have undergone heat treatment (or Thermal Enhancement as it is sometimes referred to). Heat treatment was pioneered in the 1500s and was used to transform blue Sapphires into colourless diamond imitations or improve their colour. Modern heat treatment involves heating the Sapphire to temperatures of up to 1600 Celcius. This process can lighten or darken a gemstone and improve its clarity. The steps of this process, such as the temperature used, the length of time it is applied, and the cooling down process, vary depending on the outcome that needs to be achieved. Heat Treating gemstones is a specialised skill undertaken by trained professionals.

Gemologists examine a gemstone under magnification to detect heat treatment. Gems that have undergone a heating process have:

  • Fuzzy colour areas and bands.

  • Surface pockmarks.

  • Melted facets.

  • Dot-like rutile "silk".

  • Glassy circular cracks around natural crystal inclusions.

Fluorescent reactions to ultraviolet light are also studied. Heat-treated blue Sapphire, for example, often turns a faint chalky green under short-wave U.V. light. Sapphire and Ruby are inspected under high magnification, making the above tells impossible to see with the naked eye.

Heat treatment is widely accepted because it causes a permanent improvement of the entire gemstone. The price of smaller gems within the 1-2 carat range generally isn't affected by evidence of heat treatment. Unheated Sapphires and Rubies are rare, and rarity is prized in the jewellery trade and by customers. High-quality Sapphire and Ruby (3 carats plus) that are unheated command 25% - 40% more in price than their heated counterparts.

When my customers ask me for my personal opinion of this process, my answer is this: When investing in a Sapphire, it being heated or unheated is not a deciding factor for me. I choose a Sapphire because I love the colour and the cut and because it has a WOW factor! If the Sapphire is unheated, it's a bonus. If it is heated, it is within my standard expectation for a Sapphire (or Ruby). However, if Sapphire has undergone other treatments such as Irradiation or Diffusion, this is a deal-breaker for me. I would not invest nor buy gemstone stock that has undergone "other enhancements".

If you are buying a Sapphire solely as a store of wealth (much like you might invest in Gold, Silver or Property) then I would recommend that you choose a 3 carat plus unheated gem. This is because the rarity will be an appealing factor when the decision is made to cash in your investment.

Sapphire and gemstone buyers have a right to know what they are buying, especially when spending large sums of money on a jewellery investment. This is where a Gemstone Identification Report is helpful because it gives factual evidence that a gemologist has examined your gem. A second opinion can give you added peace of mind if you have any concerns. A handful of excellent gemologists based in New Zealand are available for independent advice.


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