Heat Treatment Of Sapphire

Heat Treatment Of Sapphire
20 January

In recent months, several blue sapphires submitted to the GIA Gem Laboratory for identification have shown evidence of high-temperature heat treatment,as well as “billowy” blue color concentrations when viewed with immersion.Photo by Elizabeth Schrader.Since the advent of beryllium diffusion treatment and the trade’s
awareness of the color changes that certain chemical elements can achieve when diffused into the lattice of a ruby or sapphire,gemologists from around the world have been on the lookout for any new developments in the heat treatment of rubies and sapphires.

What appears to be the latest development, first noticed early in 2003,has taken the form of blue sapphires with unusual color concentrations. To date, we have examined dozens of sapphires that showed these unusual color concentrations as well as clear evidence of high-temperature heat treatment. With immersion,
they revealed irregular or “billowy” blue color concentrations,surrounded by pale blue or colorless areas that extended to the surface (figure 1).At the boundary between the inner and outer color concentrations,two features appear to be indicative: (1) a thin, near-colorless zone that parallels the contours of the inner color concentration; and (2) a scalloped edge that is bordered by distinctive growth and color zoning.

There has been considerable speculation about the process used to achieve the unusual color concentrations observed in this material.Although we have seen several known Be-diffused blue sapphires,
to date no significant amounts of beryllium have been found in these particular stones. Another theory that has received considerable attention involves the partial dissolution of the original sapphire during this “new” heating process and subsequent regrowth (i.e., of synthetic sapphire) over the remaining original stone.

Prior to heat treatment, the samples were placed in an alumina crucible without a fluxing agent. Photo by Matthew Hall.

To investigate this new treatment further,GIA researchers Christopher P. Smith and Matthew Hall recently traveled to Sri Lanka, where they attended experiments by Punsiri Tennakoon of Punsiri Gems, the man who developed this process. The purpose of their trip was to learn more about the procedures and conditions being used,as well as to try to determine the mechanism that might be responsible
for the particular gemological features that have been observed.

Tennakoon\'s heat treatment experiments used several samples he supplied as well as reference material that had been pre-analyzed by GIA (figure 2). The heat treatment procedure (which is proprietary to Tennakoon)had a dramatic effect on the color of most of the samples (figure 3), and initial observation suggested that the unusual color concentrations indicated above were produced in more than two-thirds of them (figure 4).

On the basis of detailed microscopic examination of the samples after treatment,Smith and Hall determined unequivocally that no synthetic sapphire growth had taken place.Furthermore, the researchers did not witness the use of any fluxing agents during the heating experiments, and no evidence of flux was present
on or around the samples brought back to the United States for further analyses.

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 become available.

When viewed in the proper direction, the heated half of the sample in figure 3 showed the same kind of unusual blue color concentration that has been noted in several blue sapphires submitted to the laboratory
in the past several months. Photo by Matthew Hall.

This 9.74-ct. sample (approximately 16.1 × 10.3 × 6.3 mm)was sawn in two just prior to the heat treatment experiment,and the half on the left retained as a control. Note the dramatic change in
color in the half on the right following treatment. Photo by Matthew Hall.

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