An Unusual Partially Non-Nacreous Pinctada radiata Natural Blister Pearl
The vast majority of pearls produced by the different Pinctada species display a lustrous nacreous surface structure. However, a small percentage of these pearls exhibit non-nacreous surfaces. GIA’s Mumbai laboratory received one very interesting example for scientific examination: a partially non-nacreous natural blister pearl, obtained from the waters of Bahrain by a noted pearl diver in March 2023.
The shell weighed 21 g and measured approximately 64.55 × 63.90 × 7.72 mm. The back of the shell showed fine layered striations toward one end and areas of massive parasite holes toward the center and hinge area. The inner surface was silvery cream in color with strong orient and held a large light cream and brownish black baroque pearl, measuring approximately 11.39 × 9.31 × 8.69 mm, attached close to the upper mantle-lip area (figure 1). This blister pearl was very unusual compared to those typically found in the Pinctada radiata mollusk. Viewed under 40× magnification, the inner shell’s brown edges exhibited columnar calcite honeycomb-like structures, while the white portion displayed the typical platy structure of fine nacreous overlapping aragonite platelets, presenting a chalky appearance near the hinge. Surrounding the central blister pearl was a yellowish brown region consisting of desiccated organic matter.
A closer look at the light cream, strongly iridescent section of the blister pearl with the distinctive platy structure looked more like “fingerprints,” distinctly separated by a delineated step feature from the adjacent black area. The black region was noticeably dull and matte and covered by a translucent layer revealing circular botryoidal surface spirals with a calcitic columnar pattern. Additionally, significant surface-reaching cracks were present in the translucent black area along the columnar structures (figure 2, left). Within the non-nacreous “cellular” structure, a layer had peeled off, accentuating the light cream calcitic step feature (figure 2, right).
Real-time microradiography imaging of the blister pearl and its host shell revealed a distinct outline showing the point of attachment of the pearl to the shell. An organic-rich core with contrasting patchy light and dark gray areas occupied almost half of the pearl’s internal structure (figure 3, left). This was surrounded by concentric growth arcs, which are classic features of natural pearl structures, similar to those observed in known Pinctada radiata pearls from GIA’s research database. In addition, significant cracks extending from the pearl to the shell and parasite tubes forming a dendritic pattern in the host shell were seen when the pearl was X-rayed in other directions. The structure indicated a natural whole pearl that had attached itself to the host shell (“Natural shell blisters and blister pearls: What’s the difference?” GIA Research News, August 26, 2019).
Due to the limitations imposed by the size of the shell, it was not possible to collect chemical data on the pearl using energy-dispersive X-ray fluorescence spectrometry. However, both shell and pearl showed an inert reaction when exposed to X-ray fluorescence, indicative of its saltwater origin. An interesting reaction was observed under long-wave ultraviolet light. The edge of the shell and part of the black area on the pearl showed a strong orangy red fluorescence, while the inner part of the shell displayed a weak blue reaction (figure 3, right). The shell was inert under short-wave UV. Similar reactions linked to a type of porphyrin pigment have been observed in partially non-nacreous and nacreous pearls from the Pteria species (S. Karampelas, “Black non-nacreous natural pearls from Pteria species,” Journal of Gemmology, Vol. 35, No. 7, 2017, pp. 590–592). Raman spectroscopy using 514 nm and 814 nm laser excitation revealed peaks at 701/704 cm–1 and 1086 cm–1, indicative of aragonite. Due to high fluorescence, no calcitic peaks were observed in the non-nacreous area. Photoluminescence spectra collected from both the shell and the blister pearl revealed three broad peaks centered at 620, 650, and 680 nm, which are characteristic of some naturally colored pearls.
Formation of natural blister pearls in the wild has always been an interesting topic of research for gemological laboratories. The studied sample is certainly noteworthy due to its size, partially non-nacreous structure, and unique reaction under long-wave ultraviolet light.
