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What's the Difference Between Unheated and Heated Sapphires?

Updated: May 4

The most frequently asked question about sapphires and rubies here at The Sapphire Merchant is, 'What is the difference between a heated sapphire and an unheated sapphire?' This article will cover everything you need to know about this process and what it means for sapphires and rubies belonging to the Corundum family.  


Unheated 4.01ct Burmese Sapphire
Unheated 4.01ct Burmese Sapphire

Should Sapphires be Heat Treated?


Heat treatment, a widely used and historic process in the industry, is applied to sapphires and rubies to enrich, remove, or even out their colour. In addition to altering the colour, heat can enhance the overall clarity of a sapphire. This is achieved by changing the chemical state of transition elements, through diffusion, or by dehydration (such as removing water from a material).


It is estimated that 95% of gem-quality sapphires and rubies worldwide 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. 


What is the Heat Treatment Process for Sapphires?


The practice of heat treatment has evolved over the years, and an exact science now exists. Modern heat treatment involves heating the sapphire or ruby rough material to temperatures up to 1600 degrees Celcius. Heat treatment is used extensively to alter the appearance of nearly all gem-quality corundum, and this process can lighten or darken a gemstone and improve its clarity. The exact 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 a trained professional and should only be attempted in a controlled environment.


Heated 2.26ct Royal Blue Sapphire - Sri Lanka Origin
Heated 2.26ct Royal Blue Sapphire - Sri Lanka Origin

Can you tell if a Sapphire has been Heat Treated?


Heat treatment resembles processes that can occur in nature, and identifying heat treatment in sapphire and ruby requires experience. A gemologist can examine a gemstone under magnification to detect heat treatment, as the corundum will have certain tells that give a clue as to whether it has undergone a heating process. These observable clues directly affect the inclusions within the internal body of the gem and can include: 

  • Fuzzy colour areas and bands.

  • Surface pockmarks.

  • Melted facets.

  • Dot-like rutile "silk".

  • Glassy circular cracks around natural crystal inclusions.


Sapphire and rubies are always inspected under high magnification, as these inclusions are much harder to observe with the naked eye. 


Fluorescent reactions to ultraviolet light can sometimes be observed in heat-treated blue sapphire, which, for example, often turns a faint chalky green under short-wave UV light. 


Even with these clues, it is only sometimes possible to determine if a sapphire or ruby has been heat-treated. However, when you shop for sapphires and rubies, you may assume that they have been heat-treated unless a laboratory report states otherwise. 


Heated 2.53ct Royal Blue Sapphire Pair - Sri Lanka Origin
Heated 2.53ct Royal Blue Sapphire Pair - Sri Lanka Origin

Are Heat Treated Sapphire Less Valuable?


Unheated sapphires and rubies, a true rarity in the gemstone world, are highly coveted by the jewellery trade, gem collectors, and connoisseurs alike. Their value is a testament to their scarcity, with high-quality unheated sapphires and rubies commanding a significant 25%—40% premium over their heated counterparts. 


When my clients ask me for my professional opinion on the heat treatment process, I answer: Investing in a sapphire, whether heated or unheated, is not a deciding factor for me. Both are valuable luxury choices. I chose a sapphire because I love the colour and the cut and because it feels right for me! If the sapphire is unheated, it's a bonus. It is within my standard expectation for a sapphire or ruby to be heated.


Sapphires and rubies can also undergo treatments like dying, filling, diffusion, and irradiation. If a sapphire or ruby has undergone other treatments, this is a deal-breaker for me. I would not invest in or buy sapphire or ruby stock that has undergone other enhancements. It dramatically reduces the gemstone's overall value. 


Unheated 4ct Cornflower Blue Sapphire - Sri Lanka Origin
Unheated 4ct Cornflower Blue Sapphire - Sri Lanka Origin

Sapphires are an Investment Gemstone


If you are buying a sapphire or ruby solely as a store of wealth, much like you might invest in gold, silver, property, or bitcoin, then I would recommend that you choose a three-carat plus unheated gem. The rarity will be an appealing factor when the decision is made to cash in your investment.


When investing a substantial amount in a gemstone and custom jewellery piece, it's crucial for sapphire and ruby buyers to have a clear understanding of what they are purchasing. A gem report from a reputable laboratory is invaluable as it provides factual evidence that a gemologist has thoroughly examined the gemstone and accurately reports the findings, including the origin of the gemstone. 


Pursuit of the Extraordinary


Here at The Sapphire Merchant, we are not your average jewellery seller - we are experts in precious gemstones, and we bring that expertise and passion to every single gemstone we select for our collection. When you buy a sapphire or ruby from us, you can be assured that all information regarding the gemstone is provided, and you are getting the best quality and value for your investment. The Sapphire Merchant has a comprehensive range of unheated and heated sapphires. Discover the sapphire that was meant for you - and only you.



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