Gemstones & Pearls/Gemology
A Diffuse Description
It's surface diffusion, but labs will avoid those terms when describing a new corundum treatment
Four of the leading gemological laboratories in the world have agreed not to use the term diffusion to describe a new corundum treatment even though it appears to involve what the trade traditionally has called diffusion. (Professional Jeweler, March 2002, p. 36). The treatment surfaced in Thailand earlier this year.
The AGTA Gemological Testing Center, the Gemological Institute of Americas Gem Trade Laboratory, the Gübelin Gem Lab and the SSEF Swiss Gemmological Institute announced the joint agreement in late February.
The labs concluded the new treatment of corundum used primarily to produce an orange component in pink sapphires from Madagascar probably is a form of diffusion, though the effects differ from diffusion treatments previously seen. Also research has uncovered new forms of diffusion that make the use of the broad term diffusion problematic when describing this particular form of the treatment.
What Lies Beneath
Gems subjected to the new treatment appear similar to other treated corundum surface-diffused with chemicals, notably blue sapphires surface-diffused with iron and titanium compounds. Some samples of the newly treated gemstones, however, feature a much deeper penetration of orange than the shallow color concentrations seen before.
The labs say routine chemical analyses of the material dont readily identify color-causing elements other diffused corundums typically have, such as titanium or chromium. Some advanced analytical testing indicates smaller amounts of lighter elements, such as beryllium, may be responsible for the induced orange hue.
Banishing the D Word
Up to now, not all labs distinguished between surface diffusion and other forms of diffusion (see Diffusion Confusion, p. 43). The labs are doing away with the term diffusion, calling it lacking both in terms of technical accuracy and descriptive purpose.
Instead, the labs will describe the new diffused sapphires as treated (orange) sapphires when labeling this variety of the corundum species in their reports. In the comments section of the reports, the labs will include the statement Indications of heating. The orange coloration of this stone is confined to a surface-related layer. The labs also agreed sapphires altered using this treatment to imitate padparadscha color cannot be labeled as padparadscha.
In a question-answer section that accompanies the labs announcement, they state that labeling the gemstones as treated (orange) sapphires will easily separate them from sapphires that dont owe their color to a surface-related layer. Adding the sentence the orange coloration is confined to a surface-related layer distinguishes this from orange color zones that may result in the interior of sapphires subjected to traditional heat treatment or may naturally occur in unheated sapphires. The comments also will help to disclose to buyers the color is likely to change if the stone is recut for any reason.
by Robert Weldon, G.G.
||At the Tucson gem fairs, Pala International, Fallbrook, CA, showed customers this sample parcel of Madagascar orange/orange-pink sapphires treated in Thailand and warned them about signs of the treatment. The company does not sell the stones.
Photo by Robert Weldon.
A word with many meanings
Some labs have used the word diffusion for a high temperature treatment in which an element, such as iron or titanium, was knowingly added to the surface of a gem to cause a color change.
Other labs more specifically described this particular type of diffusion. When we described a sapphire in which an element was clearly added to the surface, we said the stone had been surface-diffused, says Ken Scarratt, director of the AGTA Gemological Testing Center.
Experts now say there are other forms of diffusion to contend with. Heating a sapphire or ruby in an oxidizing environment like air, for example, constitutes a form of diffusion. Diffusion can be described simply as the transportation of elements into another medium the host medium, Scarratt says. Oxygen transported through a stone during high-temperature treatment could be viewed as a form of diffusion.
Another form of diffusion can occur within the gem at high temperatures internal diffusion. In this case, a gems natural inclusions melt, causing localized color changes. A good example is when rutile inclusions in a sapphire melt at high temperatures, leaving behind a rutile skeleton outlined in blue, Scarratt says.
The many forms diffusion can take may have led to confusion about how heat treatments involving diffusion were explained. But the latest development, with labs avoiding the term diffusion, concerns many. Do we need new nomenclature for these stones? asks Richard Hughes, an associate at Pala International, Fallbrook, CA. I dont think so.
The effect of such a word change in the perception of some sapphire treatments could be enormous, say critics. No longer labeling a surface-diffused stone as such, for example, could lessen the stigma attached to this form of treatment a possibility dealers of natural gems say they find abhorrent. Its a very scary blurring of the lines that were once drawn to define [certain] treated stones, says Bill Larson, president of Pala International.