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Heat treatment

One of the most common treatments applied to gemstones.

Many gemstones are heat treated, but the motive behind the treatment is rarely the same for different gemstones. But what is heat treatment? Well, simply put, many gemstones are heated by man, either before or after they are cut and faceted to enhance the stone in one way or another. It’s simply man carrying on the work of Mother Nature, but at a controlled temperature and for a controlled period of time. Heat treating is the most basic form of gemstone enhancements.

I am simplifying it a little, but if you put a gemstone in your oven at home you are heat treating your gem, whilst if you were to put it in the microwave, you would be irradiating it. It’s not quite that simple and scientists might say I was off the mark, but I like the example as a way of communicating the differences between heat treatment and irradiation. Also, often both treatments are applied: for example Green Amethyst and Morganite are normally gamma irradiated in a highly sophisticated laboratory to change their atomic structure and after the process is completed are then heat treated to transform their colour.

One of the most important questions I am asked is whether heat treating a gemstone decreases its value. Unfortunately there is no one rule here and it varies by gemstone. Aquamarine for example is virtually always heated to drive off the green hues within the gem, turning it a more pure blue. As the treatment is so common and because it is virtually impossible to detect, there is no price differential in the market. Without heat treatment there would be very few pieces of quality Tanzanite on the planet. On the odd occasion a piece is found naturally blue, then it sometimes can demand a higher price, but only if a collector is happy to pay a premium for a piece that may be less attractive than its treated brothers and sisters. However, for some gemstones such as Ruby and Sapphire (except yellow Sapphire which I will explain later), if it can be proven that they have not been heat treated, they  can sell at a huge premium.

Let’s take a look at some well known gemstones and see how heat treatment can affect the gem.

Amethyst and Citrine


If you heat treat Amethyst you can normally turn the gemstone into a Citrine. In fact the vast majority of Citrine sold today was unearthed as a lightly coloured Amethyst. When I was once in Ametista do Sul in Brazil, I witnessed first-hand how small family run gem traders transform the gemstone in fairly basic heating chambers in their garage (See photo top of opposite page).

My good friend Glenn Lehrer (there is a feature on Glenn earlier in this book) transforms his Amethyst into Citrine in a more controlled manner. He places the gems first of all into plaster of Paris, then turns on his computer controlled oven. He then slowly increases the heat in a very precise manner: once it reaches a certain temperature, he then controls the cooling process. Glenn believes that this very time consuming process is one of the reasons why his Citrine always looks so vibrant.

Yellow Sapphire

Emanuel Piat, one of the most famous gemstone dealers in Paris, once explained that whilst he would generally pay a premium for unheated Blue Sapphire, he would only ever acquire a Yellow Sapphire if it had been heat treated. He explained the reason for this is that untreated natural Yellow Sapphires can virtually lose all of their colour when exposed to too much sunlight, whereas when it is heat treated it permanently stabilises the colour.



Heat treating a Ruby will often drive off some of its brownish mask: in other words it improves the saturation of colour within the gem. This treatment was developed in Thailand hundreds of years ago and gem dealers in the city of Kanchanaburi once claimed that they treated over 80% of the world’s supply in their workshops. Today heat treatment is so common for Ruby, that to differentiate a heat treated Ruby from one that has either been fissure filled, beryllium diffused or any one of about ten different known enhancements, causes many dealers to refer to them as natural Rubies. I don’t personally agree with this, as for me a natural gemstone must have not had any treatment at all. As soon as it is treated in anyway, I prefer to call them “genuine gemstones” and not “natural”.


Most Tanzanite is unearthed as a brownish colour; there is generally speaking no blue seen. Once heated, even at fairly low temperatures, the gemstone turns into one of the most attractive blue gemstones on the planet. I once had the pleasure of spending time with Australia’s number one gem hunter Terry Coldham. For over 40 years Terry has been respected worldwide for his ongoing contribution to the coloured gemstone industry and he told me of an experiment he once did with Tanzanite. Terry explained that after returning from Arusha, Tanzania he wanted to test the story of how a bush fire had once turned a brown Zoisite into a vividly blue Tanzanite, a story that many tell of how the gem was first discovered. Terry performed a little experiment on a few loose brownish Zoisites he had uncovered on his buying trip by placing them in the bottom of his barbecue. Not only did it turn the gem into its now famous vivid blue hue, it also didn’t fracture the gem. This really surprised Terry, as normally if you rapidly heat a gem and then rapidly let it cool it will crack or at best craze (small cracks on its surface). The pieces Terry pulled out from his experiment were beautifully coloured and in every other respect identical to how they were before being put in the barbeque.

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A very basic oven used to heat treat Amethyst in Rio Grande do Sul.






Yellow Sapphire.