Blue Diffusion Treated Synthetic Sapphires

(via ICA Early Warning Flash, No.55, June 2, 1992) GIA GTL writes:

Background
Recently the GIA Gem Trade Laboratory, Inc facilities in both Santa Monica, California, and New York, received for identification parcels of faceted stones that were determined to have been diffusion-treated to produce a blue coloration. In one instance, gemological investigation revealed that all three treated stones were synthetic sapphires. In a second instance, a parcel of about 40 stones was determined to consist of approximately 2/3 natural corundums and 1/3 synthetic corundums.

Visible observations/magnification
Examination using immersion with diffused transmitted illumination revealed features characteristic of stone color enhanced through diffusion treatment; for a summary of diagnostic properties, see ‘The Identification of Blue Diffusion Treated Sapphires’ (Kane et al) in the summer 1990 issue of Gems & Gemology.

Using magnification and darkfield illumination it was possible to detect the presence of various inclusions in a number of the stones that identified these hosts as being of natural origin. In other specimens, the presence of gas bubbles proved the hosts to be synthetic. Some stones, however, exhibited no diagnostic features through microscopic examination.

Plato test
The Plato test was also used in the determination of natural vs synthetic origin. A positive Plato test, further substantiating synthetic origin, was obtained with most of the stones identified as synthetic by virtue of diagnostic inclusions, as well as with those specimens without any such internal features. It should be noted, however, that characteristic positive Plato test appearance was generally more subtle than what we are accustomed to seeing.

Ultraviolet luminescence
Examination under both long and short wave ultraviolet radiation revealed some additional potentially useful information. In many cases, the diffusion treated stones exhibited some small areas where there was no blue diffused color, most likely due to this having been removed in repolishing after diffusion treatment. Under long wave UV radiation, these areas on a number of the blue diffusion treated natural sapphires fluoresced reddish orange, a reaction often associated with colorless to light blue corundum from Sri Lanka. When exposed to short wave UV, these same areas fluoresced a chalky whitish blue, a reaction associated with some sapphires that have been exposed to high temperature treatments.

The synthetic sapphires, however, were are all completely inert to long wave UV radiation, including areas where the blue diffusion treated coloration was absent; these areas fluoresced a chalky whitish blue to short wave UV. Such luminescent reactions may be exhibited by colorless synthetic sapphires. While the short wave reactions of both natural and synthetic sapphires were similar in these cases, the effect was slightly stronger with the synthetic samples. It should be noted, however, that the strength of fluorescent reactions in natural, synthetic, and treated sapphires can vary considerably.

Discussion
Immersion used in conjunction with diffused, transmitted illumination is generally quite effective in detecting the presence of diffusion treated color in corundum. Detecting this enhancement, however, does not prove or even indicate whether the treated gem material is of natural or synthetic origin. In the above examples, a combination of magnification and Plato test were used to make this determination.

Ultraviolet luminescence provided some additional, potentially useful information. In this regard, it should be noted that the presence of the reddish orange long wave reaction may be considered a good indication that the starting material is natural; although the absence of such a reaction indicates neither natural nor synthetic origin.