HomeMajor Colored StonesRubies and SapphiresCorundum Treatment: Punsiri Technique

Approximately a year ago, unusual sapphires with "colorless rims" – especially when viewed submersed in methylene iodide – began to appear in the trade. The treatment raised speculation these might be subjected to a hitherto unknown lattice diffusion technique. Months later, the American Gem Trade Association's Gem Trade Laboratory noted it had not discovered any form of lattice diffusion in the sapphires, but concluded the stones showed diagnostic characteristics of heat treatment. 



GIA's fresh stance on corundum heat treatments follows months of intense investigations, including trips to the gem-rich island of Sri Lanka. It was from this source that the first colorless rim sapphires originated. GIA now describes the stones as exhibiting "near colorless boundaries." GIA researchers visited Punsiri Tennakoon, of Punsiri Gems in Ratnapura, Sri Lanka, the proprietor of the heat treatment facility. In analyzing his heat treatment process, researchers discovered no evidence of intentional or inadvertent diffusion of elements from an outside source. Separately, hundreds of sapphires were examined – including "geuda" sapphires, which vary from colorless to pale blue. Color concentrations inside the stones, as well as the near-colorless boundaries were subjected to various forms of infrared spectroscopy, as well as highly sensitive chemical analysis using LA-ICP-MS and SIMS machines. Before and after treatment stones were also examined. 

For sapphire dealers, the findings may provide a sense of relief, due to the potential adverse effect a different report comment could have on the perceived value of the stone. No other labs have issued reports on the sapphire treatment.

“Hundreds of non-heated and heated blue sapphires were involved in this research, including several different types of geuda, the colorless to white or pale blue starting material. Numerous tests were conducted to compare the inner blue color concentrations and the outer decolorized regions of the suspect stones.” The results of the numerous tests and visits to Sri Lanka led to the conclusion that the color concentration of these treated stones were the cause of a “specific heating regimen used by Mr. Punsiri Tennakoon of Punsiri Gems, Ratnapura, Sri Lanka.”

 

 

“In immersion, these heated blue sapphires reveal unusual, billowy color concentrations. Also indicative of this phenomenon, a narrow near-colorless boundary follows the contours of the inner blue color concentration. Photo by Elizabeth Schrader.”
 
“One hundred eighty-seven spot analyses were taken by LA-ICP-MS to map the full chemical composition of this sample. No systematic chemical variation was recorded between the inner blue color concentration and the outer decolorized region. Photo by Wuyi
Wang.”
Under immersion in methylene iodine, the color zoning is more obvious and the brownish areas are very easy to observe. Although a slight decoloration at the stone rim is possibly visible, it was not clear in this stone. We obviously have here a different case compared to the blue sapphires heated in Sri Lanka recently also at high temperature and for long period of time, following what is commonly called the "Punsiri technique" from the famous Sri Lankan burner who discovered this treatment technique.

Using dark field illumination, except for some minor fingerprints, the stone did not present any suspect features.

Using fiber optic illumination as in the photo on the right, some cloudy areas were nevertheless visible along the color zoning.


 

 

It is important to correctly orientate the stone and the fiber optic in order to see the circles correctly. Working at high magnification is also recommenced. But after some practice at 40x, the observation of these circles does not present any major difficulty.

In some cases the circles are even visible under standard dark field illumination when the stone is properly orientated.

Details of these circles using very high magnification (100x) and fiber optic illumination. We can discover here clearly that they are composed of dotted particles.

A global view on a fine cloud of circles using fiber optic illumination and high magnification (60x)

For more microphotographs please visit our inclusion study on 56 calibrated heated blue sapphires presenting similar inlcusions features.

The sapphire was not presenting any unusual feature with the spectroscope or under Long Wave or Short wave UV light. Using Hitachi U-4001 Spectrophotometer it had a similar spectrum as most metamorphic heated blue sapphires from Ilakaka in Madgascar. 

Regarding its chemistry using EDAX, it was found to have a low iron content which was similar to the data we got from the Ilakaka sapphires that we heated ourselves (See spectrumm on the right)

The SSEF reports that the stones it tested, while immersed, revealed uneven, colorless rims of about one millimeter thick, and that the colorless zones did not completely outline the stones. According to a lab alert from the International Colored Gemstone Association, the SSEF's research team suggests that the colorless rims on the stones' surfaces were caused by a technical oversight in the heating process, during which a minor degree of oxidation occurred -- causing surface de-colorization. The team also says that the colorless zones are not uniform because the stones had been treated as rough and most of the colorless areas had been cut or polished away, an observation first observed by the AGTA.

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