Heat Treatment of Gemstones

heat treatment of gemstones


Consumer Information

What is the story behind this treatment?: Heating of gemstones to enhance their color goes back centuries. It is considered a natural type of treatment since the process is basically a continuation of the processes that occur in the ground. The stone is heated to a very high temperature, which causes the inclusions to reform themselves and improve the color by making either darker, lighter, more intense, or of a different color. It has been properly stated that without heat treatment the world would be without many colored gemstones. Because colored gemstones occurring in the colors we see are very rare. But heating can take an off-color stone and turn it into a beautiful gemstone. So don’t be worried about heat treatment.




Can I wear it everyday?: Heat treatment, for the most part, is permanent. As in the case of the tanzanite shown above. This stone is a brown color coming out of the ground. But turns a beautiful blue/purple (shown at left) after heating. The few exceptions would be kunzite which will tend to fade from its medium pink color to light pink and then colorless in daylight (which is why its called the midnight stone), and yellow sapphire, which can also fade when exposed to direct sunlight.


What does this treatment do to the stone?: Basically, it rearranges the atoms in the gemstone. Sometimes, as in the case of ruby, the stone is heated almost to a point of melting, which lets the aluminum oxide in the stone reform to what is basically a new crystal structure. This lets the chromium reform to give off a better color of red. The same can apply to a type of sapphire known as gouda sapphires. These are milky white sapphires that turn blue, and account for many of the promotional quality sapphires out on the market today.


Does this treatment save money?: Oh Yeah! For instance, a 3.00 carat fine blue sapphire of natural, unheated color can sell for as much as US$20,000.00, while a heated stone of the same color can go for as little as $1500.00. Which means that you will always want to have a proper gemological evaluation for any sapphires, ruby, etc…that you buy.


What do I need to know before going shopping?: Always stay with a professional, independent jeweller with some serious gemological training. There can be a HUGE price difference in two gemstones that appear the same color and clarity due to heat treatment. And it takes someone with some good experience and good gemological knowledge to be able to identify one from the other. So stay with someone with a good reputation for quality, and someone who handles enough gemstones to be able to have some good connections with colored gemstone suppliers.


Note: Stay away from the discount stores like Service Merchandise, Sams, Walmart, etc…when shopping for fine quality colored gemstones. And for that matter…stay away from most of the Internet sellers. Colored gemstone shopping is “business for the eyes” as a good friend from Israel once told me. You can’t buy a quality gemstone from someone claiming discounts, and you can’t buy a quality gemstone based on a picture on your computer screen. Stay with a local professional jeweller. Fine quality colored gemstones are never discounted because they are too rare. And buying a colored gemstone without seeing what it looks like to your own eyes is buy what we in Texas call a “pig in a poke.” If you can’t see it, you won’t know what you got until you get home with it. And then its too late.



Gemological Information


Blue topaz before heat treatment, and after heat treatment set in jewelry.


Special Identifying Properties and Tests: There are a number of ways to identify heat treatment in sapphires. One is the easy way to find the halo effect around included crystals. This is where the inclusions caused fracturing of the stone when the heat was applied. Also (which is now taught to you GIA Graduate Gemologist…unfortunately) heated blue sapphires will generally lose their iron spectrum lines in the 4400 to 4500 region. Natural blue sapphires will generally have nice absorption lines in this region to due iron content. This is not diagnostic but I can tell you that I have not seen the absorption bands in heated sapphires. And the absence of these lines will also serve as a signpost that you might have a synthetic. But a nice strong line in the 4500 will be a strong case for a natural blue that is not treated by heat. Beyond that, check with a good reference book like GEMS by Webster, or Photoatlas if Inclusions in Gemstones by Gubelin.

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