Retailers and suppliers buying and selling “no heat” sapphires and rubies may be in for a rude awakening if gemologists determine a way to identify low-temperature heat treatment. Some stones with “no evidence of heat” written on their gem-identification reports could be reclassified as “color enhanced by heat,” turning pricey natural-color gems into not-so-pricey heated ones.
If a report includes the phrase “no evidence of heat,” it doesn’t guarantee the stone hasn’t been heated. It simply means there is no evidence the gemstone has been heated. Colored-stone suppliers should understand that such wording does not grant them license to write “no heat” on parcel papers or receipts.
“Whether or not a stone is described as ‘heat’ or ‘no heat’ has not so much to do with the temperature as it does with whether or not the labs can detect it,” writes Dr. John Emmett, Crystal Chemistry, Brush Prairie, Wash. Emmett is an expert in heat-treating gem materials and is often called on for advice and research by the major gem laboratories.
“I have been told (but never observed it), that many heat treaters do a lower-temperature burn when processing large lots of small rough,” writes Emmett. “These very low temperature burns (400 to 500 C) are usually used to coat the stones with a flux or glass or some other similar process.”
Emmett continues, “Low-temperature heat treatment does have a visible effect on some types of corundum. As you know, much of the pink sapphire from Madagascar has a bluish overtone to the color. Pink sapphires from Sri Lanka and Vietnam often have some small blue spots in them. If you heat a lot of these stones at once, you will note that a few of them lose all or part of the bluish color at 500 C.”
At present, there is no known way to tell that a gem’s color has been changed by heat treatment. Inclusions that are typically altered when subjected to high-temperature color enhancement show little or no alteration from low-temperature treatment. “I don’t think that we should ignore low-temperature processes, as in some cases they can substantially increase value,” says Emmett, who is working on developing techniques to detect low-temperature heat treatment in gems.