Over the past several months, there has been considerable concern in the gem and jewelry industry about blue sapphires with unusual color zoning. Typically these blue sapphires have a dark blue billowy core that is outlined by a fine, colorless band and then surrounded by a light blue to near-colorless zone that extends to the surface of the stone. Both the AGTA Gem Testing Center and GIA Gem Laboratory have studied numerous samples of this material with a variety of methods. Following is an update on their research to date.

Background

Gemologists at both laboratories began to notice sapphires with this unusual zoning in early 2003 – first in small numbers and then in sizeable quantities. In overall appearance, they were generally medium blue in color and would be considered better-quality commercial stones.

Sapphires subjected to this treatment typically have a dark blue central zone that is separated from a paler outer zone by curved edges, as can be seen by these stones at 10x magnification with immersion. Photos by Shane McClure.

After extensive examination of these stones in the two laboratories and consultations with members of the trade, it was established that these blue sapphires originated from a single heat treating operation in Sri Lanka. The proprietor of this operation came to New York to share samples and information about his heating process. Among the samples he provided for examination were sapphires before treatment and several categories of treated stones.

Although this visit and the samples were helpful, they did not provide clear answers as to the cause of the unusual color distribution or the mechanisms involved in creating it. Possible scenarios included the presence of a synthetic overgrowth, the use of a diffusion process, the reaction of a particular type of corundum to heat, or even a new treatment process.

Analysis to date

In parallel, both laboratories have been purchasing samples on the market that show this color distribution. Many of these have been sawn into slabs to facilitate careful analytical measurements. In addition, heating experiments are being conducted in the U.S. to reproduce the unusual color distribution. Both of these programs are ongoing.

Plans are currently under way for a second trip to Sri Lanka to conduct more extensive treatment experiments. It is hoped that these experiments and continued experimentation with heating in the U.S. will lead to a better understanding of what is causing both the color and the unusual zoning in these sapphires, and ultimately provide clear reporting on them.

Findings to date

• After instrumental measurements and visual observations on hundreds of stones, both laboratories feel that synthetic overgrowth can be ruled out as the cause of the color zoning.
• Testing for a number of light elements and for more typical chemical elements that have been diffused into corundum has not, to date, revealed any foreign elements that can be designated as causing the unusual color zoning.
• Both GIA and AGTA note on their laboratory reports that these stones show indications of heating, but follow that with a comment that further research is necessary to fully characterize them.

Although the exact cause of the unusual zoning is not yet known, both laboratories feel that the aggressive research plan in place will provide conclusive answers in the next few months.

For further information on this research, read Gems & Gemology, Winter 2003, pp.319–321; visit www.gia.edu, GIA Insider, Dec. 19, 2003; and visit AGTA at www.agta.org, AGTA-GTC Lab.

Feb. 3, 2004