Deep Diffusion Treatment Of Corundum

(via ICA Early Warning Flash, No. 32, February 16, 1990) GIA writes:

General background
At one of the gem shows held in Tucson, Arizona, during February of 1990 one exhibitor was offering for sale blue sapphires which were reportedly enhanced with a ‘deep’ diffusion treatment. The authors of this report were shown two plastic bags of reportedly treated material, each containing an estimated several hundred carats of these treated stones. A number of these were purchased for investigation.

Gemological properties
Eleven treated stones were examined. Basic gemological properties—refractive index, birefringence, optic character, Chelsea filter reaction—were all consistent with those reported in the literature for natural blue sapphires.

The color of the stones was fairly uniform, being a medium dark slightly violetish blue. All were very transparent.

None of the stones exhibited any absorption features of the type association with iron-bearing blue sapphires. Three of the stones, however, exhibited a bright fluorescent line centered at 693nm. It is interesting in this regard that the vendor’s promotional flier states that ‘….all stones treated are genuine Ceylon sapphires…’

Key identifying features
Under longwave ultraviolet radiation all but two of the stones were inert; these same stones all fluoresced a weak to moderate, chalky yellowish green to short wave ultraviolet. The other two stones fluoresced a weak to moderate pinkish orange to longwave ultraviolet; the shortwave reaction was similar but weaker in intensity. None of the stones exhibited any phosphorescence.

Magnification and darkfield illumination revealed some features associated with corundum that has been subjected to high temperatures: diffused color banding, broken ‘dot-like’ acicular inclusions, melted included crystals resembling ‘snowballs’ with surrounding spatter halos, and superficial sintering in surface pits. It should be noted that these features were, in general, minor.

Examination using diffused transmitted light without immersion or magnification showed color concentrations along facet junctions and outlining of the girdle edge. Also noted was some variation in color from one facet to another.

Using immersion and diffused transmitted light without magnification the color concentrations along facet junctions and the girdle as well as the uneven facet-to-facet coloration were again noted. In many cases these features were significantly more obvious when immersion was now used.

Suggestion
Diffusion treated sapphires are now being made increasingly available in the market. It this becomes important to use diffused transmitted illumination and immersion in the routine examination of corundum.