Any gem with elongated needle inclusions or a fibrous structure can produce chatoyancy when cut as a cabochon with the proper orientation.
The Carlsbad laboratory recently received for identification a 30.45 ct translucent, saturated blue oval cabochon with an extremely sharp eye. Standard gemological testing gave a spot RI of 1.56; the stone showed a moderately strong blue fluorescence to long-wave UV radiation and a weak blue reaction to short-wave UV.
While no particular gem came to mind during initial observation, the combination of saturated blue color and a sharp eye led us to suspect something too good to be true. This suspicion was confirmed during microscopic examination, which revealed a uniform fibrous structure and a blue coating on the top portion of the cabochon.
Decolorized areas related to surface damage provided further evidence of a superficial color coating. The base of the cabochon had a colorless coating that was easily observed, as a portion of it had delaminated from the substrate. The underlying material was very soft and easily scratched by a metal probe.
Raman spectroscopy of the top and bottom coatings gave results consistent with a polymer resin, and the substrate material was identified as gypsum. This was the first example of a color-coated cat’s-eye gypsum examined in GIA’s laboratory.
Image: This 30.45 ct blue oval cabochon proved to be a coated cat’s-eye gypsum (top, photo by Robison McMurtry). A blue color coating was clearly visible on the surface of the gypsum cabochon (bottom, photomicrograph by Nathan Renfro; image width 1.08 mm).
Courtesy of GIA (Gemological Institute of America) © 2012.