The trend toward the unique is leading many suppliers and retailers to search for untreated ruby, sapphire, and emerald. So it wasn’t a surprise that the annual Accredited Gemologists Association’s Tucson conference saw members hunting for inclusions, natural gem identification clues, signs of geologic origin, and remnants of micro-enhancements. Presenting the details of what to look for in colored gems were Shane McClure, gem identification director for the Gemological Institute of America; John Koivula, of the American Gem Trade Association Gem Testing Center; and Dr. Michael Krzemnicki, from the Swiss Gemmological Institute (of SSEF).
Nick Del Re, of EGL USA and Canada, led the group in examining diamonds for new laser enhancements.
It’s becoming nearly impossible for a labortory to be 100 percent certain that a stone has not been heated, McClure explained. On identification reports, the wording “No indications of heat” reflects the laboratory’s limitations. “We don’t say ‘natural color,’” says McClure. “We haven’t for years.”
Heat treatment is difficult to determine with certainty because three different temperature ranges are used to treat gems, and inclusions react differently to them.
Low-temperature treatment uses temperatures of up to 500°C and is most commonly used for Madagascan sapphire. Most inclusions show no reaction to this temperature.
Moderate temperatures range from 500°C to 1,250°C, which can damage rutile, present in sapphire and ruby.
High-temperature treatment ranges from 1,250°C to 1,700°C. This is typically used to clarify (by dissolving rutile) and color-enhance (by moving iron and titanium molecules) Sri Lankan geuda sapphire.
Extreme heat, categorized as anything over 1,700°C but which rarely exceeds 1,850°C, is used for Mong Hsu ruby and beryllium treatment of corundum. At temperatures above 1,850°C, crucibles break and furnaces begin to break down.
Some points to note:
Blue halos in an orange sapphire indicate very high- temperature heat treatment.
Zircon crystals seen in Sri Lankan and Madagascan sapphires can resist fairly high temperatures. Small multidirectional halos form naturally around zircon crystals.
“Snowballs” are proof of higher temperature ranges.
East African corundums tend to survive heat better than others. Hematite platelets in Umba River sapphires can withstand all but very high heat treatment.
Metamorphic stones are already heated—by Mother Nature. They were exposed to temperatures ranging from 800°C to 1,000°C.
Color zoning can be helpful. Natural-color zones follow the growth structure of the crystal.
“People hope for certain origins,” says Koivula. “Some labs do origins, some labs don’t.” But Koivula questions the importance of “where something came from.” Mother Nature doesn’t arrange gem deposits around political boundaries. It’s geological boundaries that are important.
“Determining geology is easy,” says Koivula. But geologic determination may not pin down origin as accurately as some people might like, he adds. And heat treatment alters the ability to determine origins.
Chemistry is too volatile to use, since the chemistry of a gem locality can change during the growth cycle of the gem. “So we fall back on inclusions and what the interior of the stone looks like,” Koivula says.
For example, noting similar geological formations, one can say that a particular stone is a Thai stone—but it also could be an Australia one. Carbonates appear in Burma, and also in Vietnam and Afghanistan.
In the end, says Koivula, “It’s not just chemistry, it’s not just inclusions. It becomes your best guess—unless you’ve mined it yourself.”