Iridescent Imitation Pearl
An unusual brown oval specimen weighing 8.03 ct was recently submitted to GIA’s Bangkok laboratory for pearl identification. At first sight, the material resembled a pen pearl due to its dark color and non-nacreous appearance (figure 1). Upon closer inspection, however, it lacked the cellular structure characteristic of such pearls (N. Sturman et al., “Observations on pearls reportedly from the Pinnidae family (pen pearls),” Fall 2014 G&G, pp. 202–215). It instead displayed a wispy, fibrous-like pattern and mosaic patches of iridescence, resembling a lava cave sheathed in spiderwebs (figure 2, left). Under fiber-optic illumination, the iridescence phenomenon was visible within the body and varied as the item was moved and turned in the light (figure 2, right).
Observed externally were both a banding feature (commonly found in shell-related materials) and numerous lines across the base indicating it had been worked and altered from its original form. Real-time X-ray microradiography also revealed a distinct banded internal growth structure and parasite tubes. Raman spectroscopy identified this imitation pearl as calcite, and its bodycolor was likely treated. All this evidence, combined with energy-dispersive X-ray fluorescence analysis, indicated that the material was not a whole pearl but likely fashioned from the shell of a marine organism.
Although GIA has encountered a variety of imitation materials, this is the most fascinating specimen seen by the author.
