Calcite Present in a Pearl from Pinctada maculata
Pinctada maculata pearls, also known in the trade as “pipi pearls,” form in shells considered to be the smallest of all the Pinctada species. The shell size ranges from 2 to 6 cm and produces pearls ranging from 1 to 4 mm. However, some rare examples may reach up to 9 mm (Spring 2014 GNI, pp. 89–90). Most pearls from this mollusk exhibit a yellow to “golden” color range, yet some are white to cream, orange, and gray. Round to near-round pearls are quite typical of the species, but other shapes may be found, as with all pearls (N. Nilpetploy et al., “The gemological characteristics of pipi pearls reportedly from Pinctada maculata,” Winter 2018 G&G, pp. 418–427). They are distributed throughout French Polynesia and the Cook Islands and found in shallow waters and depths up to 20 meters. GIA’s Mumbai laboratory had the opportunity to examine a quantity of these pearls, which were reportedly sourced from known divers who fished them from wild P. maculata mollusks off the Cook Islands. The parcel contained 306 natural pearls of various sizes and shapes, with measurements ranging from 2.87 × 2.69 × 2.55 mm to 8.34 × 3.25 mm.
Real-time microradiography (RTX) showed a variety of internal structures expected for natural pearls. Externally, most samples showed the typical nacreous overlapping aragonite platelets (platy structure) and surfaces that ranged from clean to moderately blemished. However, one near-oval, bicolor white and cream pearl, weighing 0.29 ct and measuring 3.75 × 3.55 mm (figure 1), revealed a very interesting surface structure under 40× magnification. The top part of the pearl exhibited a mosaic, or network of columnar calcite structures in a cellular form (hexagonal and clustered), creating an uneven surface. This was similar to the appearance of non-nacreous pen pearls (N. Sturman et al., “Observations on pearls reportedly from the Pinnidae family (pen pearls),” Fall 2014 G&G, pp. 202–215). The bottom part had a combination of both: areas of nacreous overlapping aragonite platelets (the circular ring-like area around the base) and areas of columnar calcite structure (figure 2). The surface was unique among those observed in the lot, as it exhibited both polymorph forms of calcium carbonate (aragonite and calcite). To confirm the non-nacreous cellular areas observed, Raman analysis using a 514 nm laser excitation was carried out on three spots of the pearl’s surface (spots 1 and 2 with cellular structure and spot 3 with platy structure). Spots 1 and 2 showed peaks at 280 and 712 cm–1, respectively, indicative of calcite. Spot 3 showed a doublet at 701 and 704 cm–1 and a peak at 1085 cm–1, both characteristic features of aragonite (figure 3).
RTX imaging revealed an organic-rich fine acicular feature with no central core, associated with some concentric growth arcs (figure 4, left). X-ray computed microtomography (μ-CT) showed a large central fine acicular feature with a small core showing its calcitic nature, surrounded with thick, dense organic-rich concentric growth arcs (figure 4, right). The internal structure of the pearl matched with the expected columnar structure observed on the surface, as noted in previous research reports.
Energy-dispersive X-ray fluorescence spectrometry revealed a low manganese level of 16 ppm and a higher strontium level of 1286 ppm. Optical X-ray fluorescence was inert, and both results are characteristic of the saltwater environment in which the pearl formed. The pearl showed a moderate bluish green reaction under long-wave UV radiation and a much weaker bluish green reaction under short-wave UV. It was also examined by the ultra-short UV wavelength (230 nm) of the DiamondView, which revealed a clear bluish reaction with fine cellular hexagonal columnar features typically indicative of calcite.
Spectroscopic examination confirmed the presence of both calcite and aragonite. The top and bottom portions of this P. maculata pearl exhibited a combination of calcite and aragonite surface features aligned parallel to each other. Such external features are not typically observed in pearls from this mollusk, based on the extensive samples GIA has examined globally, which makes this pearl rare and interesting despite its lack of visual appeal.
