Professional Jeweler Archive: Corundum Conundrum

March 2002

Gemstones & Pearls/Gemology

Corundum Conundrum

A new and different color-altering corundum treatment surfaces in Thailand. Experts are unsure whether diffusion is involved

In January, the American Gem Trade Association Gemological Testing Center reported seeing new treatments in sapphires (padparadscha and other colors) and some rubies from Thailand. The lab says treatment results in a yellow or orange coloration that penetrates the gems at varying depths. The lab believes the treament involves diffusion and very high heating, though further tests are under way.

Not all labs agree with AGTA’s initial assessment. The Gemological Institute of America, which released a preliminary report in late January, says its researchers aren’t comfortable calling the process diffusion. The Gem and Jewelry Institute of Thailand Gem Testing Laboratory also examined the treated gems and says diffusion is not a factor.

Diffusion enhancement involves a high-temperature process in which trace elements of coloring agents such as iron, titanium, magnesium, manganese or beryllium are diffused into thin surface layers of the stone, causing a perceived change in color. AGTA GTC Director Ken Scarratt says the characteristic sign of diffusion is a thin rim of surface layer of distinct coloration that envelops a stone, which he has noted in the sapphires and rubies he’s examined.

GIA Assessment

The GIA report, which appeared in GIA’s online publication GIA Insider, Vol. 4, No. 3, discusses a study of 48 stones. Authors Shane McClure, Tom Moses, John I. Koivula and Wuyi Wang say the stones ranged from 0.34 carat to 3.53 carats and originated from Madagascar and Songea, Tanzania. The post-treatment corundum ranged from orange to pinkish-orange, orangey red and red-orange. GIA researchers found an orange layer along the gems’ surface and at varying depths, as described by AGTA GTC, and concurred the stones showed evidence of high-temperature heat treatment. However, they added “the color layer in some of the stones extended deeper than any diffusion treatment we have seen in the past.”

GIA conducted cross-section examination of some samples and found penetration of the orange zone was variable in the gems reportedly from Madagascar, with some showing an 80% depth of color penetration. Gems reportedly from Songea showed an even orange coloration throughout, with original material said to have started out green before treatment.

“Although we believe the color in these stones is being produced by a form of high-temperature heat treatment, we do not currently know the exact treatment method(s),” says the report. “At this time, we question whether this new treatment should be classified in the same category as the surface diffusion-treated stones we reported on in the past.”

GIA is studying trace elements known to cause color, but adds it’s seeking the use of instrumentation with more sensitive detection limits. GIA says its Gem Trade Laboratory identification reports will indicate the treatment, when appropriate, with a phrase such as: “The orange color of this stone is confined to a near-surface layer.”

GIA stresses its report is preliminary and promises to make more information available as its study continues.

Thai Lab Reports

The Gem and Jewelry Institute of Thailand Gem Testing Laboratory says its examinations of the treated gems indicate heat treatment (perhaps combined with pressure), but not diffusion. Subsequent chemical analysis of the gems revealed trace concentrations of iron. The lab’s analysis points to the oxidation of iron with oxygen, an iron transfer mechanism that causes the color change, even if only minute amounts of iron are present in the untreated gem. Color change can vary from skin depth to total penetration, depending on the duration of the enhancement process.

Emmanuel Fritsch and George Rossman described an initial study of an iron transfer mechanism in some pink sapphires in the Spring 1988 issue of Gems & Gemology, GIA’s quarterly publication.

Identification Is Straightforward

“The good news is that the treatment appears to be quite identifiable for a number of reasons,” says Scarratt. “The high-temperature treatment causes inclusions to have identifiable features, such as glassy residue and expansion fractures that occur during very high heating.”

– by Robert Weldon, G.G.

Natural Madagascar pink sapphires (left) and orange padparadscha-like colors after treatment (right).

Photo by AGTA Gemological Testing Center.

Heat-treated sapphires including diffusion-treated gems, show expanded stress fractures around zircon inclusions. The concentric glassy borders around the fracture develop as a result of the extremely high temperatures.

Photo by AGTA Gemological Testing Center.

Copyright © 2002 by Bond Communications