Corundum Treatments

Corundum gems receive more types of treatments than any other stones. Learn how to test rubies and sapphires for signs of these enhancements.

13 Minute Read

Heat treatment is accepted as a common practice and has little effect on corundum values. In contrast, other treatments have considerable impact. Some can reduce the worth of rubies and sapphires drastically, by 50% to even 95%.

Some of the treatments are very subtle, invisible even with a loupe or microscope. To make matters worse, many treated rubies and sapphires will also have natural inclusions. You have to examine them very carefully to detect any treatment.

Due to the impact some corundum treatments have on value, finding evidence of these procedures can be critical.

Heat Treatment

Most corundums contain rutile inclusions in the form of long, thin crystals, so fine that gemologists refer to them as “silk.”

silk in untreated sapphires
stress fractures in heated corundumRutile melts at a temperature slightly below that of the surrounding corundum. By heating it to the proper temperature and with a controlled cooling period, the rutile becomes absorbed into the crystal lattice. This improves the clarity and often the color of a gem. The process can also enhance the silk, which improves asterism.

Almost all rubies and sapphires receive heat treatment. It sometimes leaves stress fractures or halos around included crystals. You may also find partially dissolved silk, distinguishable…

Donald Clark, CSM IMG

The late Donald Clark, CSM founded the International Gem Society in 1998. Donald started in the gem and jewelry industry in 1976. He received his formal gemology training from the Gemological Institute of America (GIA) and the American Society of Gemcutters (ASG). The letters “CSM” after his name stood for Certified Supreme Master Gemcutter, a designation of Wykoff’s ASG which has often been referred to as the doctorate of gem cutting. The American Society of Gemcutters only had 54 people reach this level. Along with dozens of articles for leading trade magazines, Donald authored the book “Modern Faceting, the Easy Way.”

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