No Indication of Diffusion?

By Gary Roskin, G.G., FGA, Senior Editor -- JCK Online, 4/1/2004 2:00:00 AM

Diffusion has nothing to do with the new enhanced blue sapphires seen recently in the trade. That's the view of Dr. Henry Hänni of the Swiss Gemological Institute (SSEF) in Basel, but others aren't so sure. In a combined press release, directors of the AGTA Gem Testing Center laboratory (GTC) and GIA's Gem Trade Laboratory (GTL) said, "At present, we cannot state conclusively the role that lattice diffusion plays in this peculiar color phenomenon or which elements may be involved. However, we are continuing our research into the exact mechanisms responsible for these unusual color concentrations and will release our findings to the trade as they are available."

"These suspect blues first came to the attention of gemologists in December 2002," recalls Richard Hughes, sapphire expert at Pala International, Fallbrook, Calif. It was GTC director Ken Scarratt who first noticed the suspicious color pattern on a stone submitted for testing.

But on Jan. 15, 2003, the International Colored Stone Association (ICA) made the following statement in a press release: "Two of the world's leading gemological laboratories—SSEF-Swiss Gemological Institute of Basel, Switzerland, and the Gemological Institute of America, of Carlsbad, California—have both reported that they found no evidence of diffusion in a batch of heated blue Sri Lankan sapphires that were heat-treated by the same source. This followed recent reports in the trade media, in which it was suggested that a new heating process for blue sapphire from Sri Lanka might involve 'lattice diffusion.' "

Rather than settling the issue, that statement raised a key question: If diffusion didn't cause the "peculiar color phenomenon" then what did? Hughes notes that some in the trade believed that the light rims were synthetic corundum overgrowths on natural cores, probably developed during high-temperature heat treatment. Others felt that the light rims were indeed caused from outside-in lattice diffusion of color bleaching agents such as beryllium, magnesium, or lithium during the heating process.

Hänni points out that if the outer colorless zone were caused by a diffusion process in which foreign elements were introduced during heating, it could trigger a negative reaction similar to that observed in the beryllium-diffusion-treated corundum that came into the market in 2001 and 2002. But Hänni doesn't believe that's the case with the more recent heat-treated blue sapphires. He says that these sapphires, which range from 4.56 cts. to 11.11 cts., undergo only heat-treating. He believes the rough is heat-treated, then the stone is cut, leaving the unusually shaped colorless outer zone. "In our present case we are thus dealing with sapphires that sustain parts of surface-related fading zones," says a statement from SSEF.

Hughes says the controversy raises other questions: Since sapphires have been treated at high temperatures for close to three decades, why weren't such features found before? Would a variation of heating time and temperature be sufficient to produce such unusual features? Or had the features been seen all along but ignored? "Is the reason they now attract attention because the recent beryllium business has gemologists wielding their weapons with hair triggers, ready to blast away at the slightest movement in the bushes?" Hughes asks.

Hänni believes the colorless regions on the rim are the result of a technical oversight in the heating process. "Towards the end of the heating process, the pump for protective gas is stopped, and some air containing oxygen enters the furnace," he says. According to Hänni, this results in a minor degree of oxidation, resulting in the decolorization of the surface-related layer of the stone. "We do not see evidence of diffusion of a foreign element, as in the case of the beryllium-treated orange and yellow sapphires," Hänni says in a report.

Hughes points out, however, that "others have suggested that a defective heating process is probably not the case, that to bleach color from the rim, something would have to be diffused in."

Hughes continues, "According to current knowledge, hydrogen would diffuse in far too fast for the above scenario and oxygen far too slow. That's the theory, anyway, but as we learned from the beryllium business, theories will need to be tested by rigorous experiments."

Although GTL gemologists have determined that no synthetic sapphire growth took place and there is no evidence that the colorless rims resulted from a bulk diffusion treatment (a treatment involving outside-in diffusion of coloring agents such as beryllium or lithium), they stress that this conclusion is only preliminary. They are still unsure whether lattice diffusion is involved.

Hughes gives the industry high marks for its reaction to the discovery: "Without question, the recent cooperation between Punsiri Tennakoon [the Sri Lankan heat treater creating this material] and the world gemological community is the right way to solve a serious problem. This is in contrast to the earlier beryllium fiasco, where burners in Thailand tried to hide what they were doing, with disastrous consequences for that nation's entire gem industry."

Hughes continues, "In the current case, the outcome is looking far better. Perhaps these blues can now begin to come in from the cold. This is a plus for all parties, but particularly for the science of gemology, showing as it does that gemologists are not 'out to get' anything except the truth."