(via ICA Early Warning Flash, No.46, August 16, 1991) GIA GTL writes:
Description
The stone is a transparent oval mixed cut weighing 0.98 carat and measuring 6.80x4.90x3.36mm. It is medium yellowish orange in color which to the unaided eye appears uniform in distribution.
Gemological properties
Standard gemological testing identify the stone as a natural corundum. It exhibits no distinct absorption features when examined with a desk-model spectroscope and is inert to both long and short wave ultraviolet radiation. Interestingly, it exhibits no pleochroism when viewed through a calcite dichroscope, something that would be expected in a corundum of this hue and depth of color.
Magnification
Diffused transmitted lighting reveals that the stone has been surface coated. Irregularities in the coating—scratches and pits on pavilion facets as well as abrasions on facet junctions---indicate that this is an essentially colorless stone. Examination in surface reflected lighting reveals a predominantly purple iridescence on pavilion facets.
Additional testing
The ultraviolet visible absorption spectrum was found to be similar to that of natural color yellow sapphire with features related to Fe3+. A qualitative chemical analysis performed by EDXRF reveals the presence of iron as the dominating trace element, with small amounts of potassium, calcium, titanium and gallium. Neither ultraviolet visible absorption spectroscopy nor X-ray fluorescence helped to detect or characterize the coating.
Discussion
The microscope features of this stone show it to have been coated, with the coating being responsible for both the apparent body color and superficial iridescence. It is important to note that this is a surface coating and not a diffusion treatment.
‘Aqua Aura’ is the trade name used for a type of coated gem seen for some time now. This consists of such materials as unfashioned rock crystal specimen as well as faceted rock crystal and colorless topaz to which a thin layer of gold has been applied. The treatment produces a greenish blue apparent body color (the transmission color of the gold coating) and superficial iridescence. Treated gems of these type exhibit microscopic features like those described above. It is possible that the corundum described herein has been subjected to a similar coating but of a different substance.
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