Diffusion Treated Corundum In Pink To Red To Purple Color Range

(via ICA Early Warning Flash, No.69, May 14, 1993) GIA GTL writes:

Background
The diffusion treated stones described herein were provided for examination by United Radiant Applications, a Southern California-based firm that has been involved in the commercial production of blue diffusion treated sapphires. The faceted specimens, which included 27 stones in the red to pink to purple color range, were made available so that their gemological properties and identification criteria could be documented prior to any commercial release.

Visual appearance
Face-up some of the stones appear uniform in color, while other exhibit uneven color distribution, the latter apparently due to an absence of color on some pavilion facets.

Magnification
A number of features previously documented with blue diffusion treated sapphires were noted in these stones. These include uneven coloration from one facet to another, color concentrations in surface-reaching cavities and fractures, and color reinforcement of facet junctions, although the latter was often significantly more subtle than what we have encountered with blue diffusion-treated stones. Also noted was a type of surface and near surface damage, including minute spherical voids that we had not previously documented in blue diffusion treated sapphires.

Refractive indices
Values were generally higher than those normal for corundum, including some reading over the limits of the conventional refractometer (1.80 +).

Pleochroism
Some stones exhibited atypical dichroism, including a brownish yellow dichroic color.

Short wave UV luminescence
The majority of the stones showed a patchy bluish white luminescence to this wavelength at the surface that was sometimes confined to specific facets or groups of facets.

Absorption spectra
These were generally consistent with those of both natural and synthetic corundums of comparable color, although some absorption features were less pronounced.

Discussion
The diffusion treated corundums described herein are not difficult to identify. Key features include unusually high refractive index readings, atypical dichroism and UV luminescence, patchy surface coloration, color concentrations along facet junctions, and spherical voids just below the surface.