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The mechanism which allows the modifications is unleashed by raising of the temperature.In the majority of cases, this allows chemical exchanges at atomic levels between the gemstone and its surrounding atmosphere.Therefore, the most interesting aspects will involve color and clarity modifications.

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Exposing gemstones to high temperature often causes distinct modifications.

Inclusions (several are often present) could suffer for distinctive alterations that, sometimes (rarely) could led to irreparable damages. The sharpness of the edges and the presence of rutile needles led to exclude high temperature exposition. Scarani Click to View Full Size Another example of diagnostic evidence is the morphological modification of the included crystal shape; well defined, with straight surfaces and distinct edges in unheated stones (photo 8 ), they will appear blemished, more rounded and free from clear edges in treated stones (photo 9), in a direct proportional grade to the reached temperature.

Geuda sapphires are commonly near-colorless and the clarity is heavily affected by the abundant presence of rutile in clouds or silk bands shapes, this is in common with Mong-hsu rubies which have almost always distinct blue zoning affecting the color.

In both cases heat treatment allowed a terrific improvement of color (enrichment in Geuda sapphires, photo 10-11 and removing of the blue component in Mong-hsu rubies, foto 12) and clarity by silk (rutile) exsolution.

Nowadays many mines providing for high quality gemstones are sterile or in the way of depletion while demand for such gemstones remains very high so, this has led to try to make profitable low grade materials by using enhancement treatments.

It’s the typical case of the so-called “Geuda” Sri-lankan sapphires (the name comes from singalese word used to describe grey-bluish low grade material) flooded the market in the second mid of 70’ and of Mong-hsu Burma rubies appeared in huge quantity on the market in early 90’.Nevertheless, we must say, that until the first studies were preformed by Dr. Photo: Len Rummel, 2004 Click to View Full Size But How is common heat treatment performed?Although there are many variations of the procedure and a number of gems are involved, we can summarize by saying that there are two main elements involved: the temperature and the environment (atmosphere composition and pressure) in which it took place.C.; many examples were also recovered from Egyptian tombs, including treasures found with Tutankhamen (Circa 1300 B. In more than 4000 years, the procedure hasn't substantially changed!This will be apparent when you have a look at some elementary ovens that are still operational nowadays in the Chanthaburi area, Thailand and Cambodia (photo 1).There are many different effects which can occur when a stone is heated: from darkening to a color change/improvement, from a structural variation to a crack.

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