January 29, 2002
GIA Releases New Corundum Treatment Report
Citing the controversy about a new corundum treatment, a research team at the Gemological Institute of America, Carlsbad, CA, released a preliminary report outlining its findings.
The report, which appeared in GIA's online publication GIA Insider, Vol.4, No. 3, discusses the study of 48 stones said to have been enhanced in a process that adds an orange coloration to a variety of sapphires and some rubies through an as-yet-unknown process. Authors of the report are Shane McClure, Tom Moses, John I. Koivula and Wuyi Wang.
While some gemologists and labs describe the process as a form of surface diffusion, GIA distanced itself from an exact description. "Although we believe that 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) being used," says the report. "At this time, however, 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 researchers say the stones studied were 0.34 carats to 3.53 carats and purported to originate from Madagascar and Songea, Tanzania. The post-treatment corundum ranged from orange to pinkish-orange, orangey red and red-orange. GIA's researchers found a similar orange layer along the gems' surface, and at varying depths, previously described by the American Gem Trade Association's Gemological Testing Center, New York City. They concurred with AGTA the stones examined showed evidence of high-temperature heat treatment (which is necessary for the diffusion process), but noted: "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 quite variable in stones represented as being from Madagascar, with some showing an 80% depth of color penetration. Stones represented as being from Songea, Tanzania, showed an even orange coloration throughout, with original material said to have started out green before treatment.
GIA says it is studying trace elements in the gems known to cause color, but adds it is seeking the use of instrumentation with more sensitive detection limits. GIA says the Gem Trade Laboratory identification reports will indicate the treatment, when appropriate, with such terms 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 to the trade as it continues to study the issue.
by Robert Weldon, G.G.