Labs and Experts Question Cause of Corundum Color Change


January 24, 2002

Labs and Experts Question Whether Diffusion Causes Corundum Color Change

The American Gem Trade Association Gemological Testing Center reported seeing new treatments in sapphires (padparadscha and other colors) and some rubies coming from Thailand. It says it believes the treatment involves diffusion combined with very high heating. The AGTA lab is conducting further investigations ("Padparadscha Treatment Surfaces," "Corundum Diffusion Treatment Includes Rubies"). Diffusion enhancement uses a high-temperature process in which trace elements of coloring agents, such as iron, titanium, magnesium or manganese are diffused into thin surface layers of the stone, causing the change in perceived 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. However, not all labs agree with this assessment.

The Gemological Institute of America's Gem Trade Laboratory, which is also investigating the new treatment, says its researchers are not comfortable with calling the process "diffusion." GIA is expected to release its preliminary findings about the treatment as early as tomorrow. GIA's Shane McClure says these gems don't show the same uneven coloration, or concentrations of color along facet junctions, from one facet to another as diffusion treatments in the past have. "In any case what we are now seeing, should it prove to be diffusion, is not the typical diffusion we have seen in the past," he says. "Thai people we have spoken to are adamant no chemicals are being added in the treatment."

The Gem and Jewelry Institute of Thailand Gem Testing Laboratory has examined several "padparadscha-like" stones as well, and says its indications point to heat treatment (perhaps combined with pressure), but not diffusion. They say subsequent chemical analysis of treated stones reveals trace element concentrations of iron. Their 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 stone. They say the color change can vary from skin depth to total penetration of the stone, depending on the duration of the enhancement process. Initial study of an iron transfer mechanism in some pink sapphires was described by Emmanuel Fritsch and George Rossman in the Spring 1988 issue of Gems & Gemology, GIA's quarterly publication.

Yianni Melas, G.G., a gem buyer with Swarovski Co. recently returned from Bangkok, Thailand, says he's also now convinced no diffusion process is occurring in the gems he's seen. Melas says information gathered from contacts and personal examination of the stones instead points to the same high-temperature, iron oxidation transfer described previously, which can cause the altered colors to be skin deep or penetrate the stone entirely. "I have had [these] sapphires sliced like a loaf of bread and have seen thin to thick rims of color, as well as total color penetration," he says.

Melas says the orange coloration seen in many of the stones is basically the effect yellow (from the iron oxidation) has on pink. "So white sapphires can turn yellow, light pinks can turn bright orange depending on the degree of penetration," he says. "And medium pink stones from Madagascar can turn either medium-orange or a padparadscha color, also depending on the degree of penetration. Melas says the same holds true for the rubies that show a rim of orange color. "The dark Thai rubies treat well this way, possibly because they are iron-rich. Rubies from Burma are not having good results – less than 20% success rates," he says. "But the Madagascar material heat treats best of all – up to an 80% success rate," he reports.

– by Robert Weldon, G.G.

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