Pearls from the Placunidae Family (Windowpane Oysters)
Nacreous pearls are commonly constructed from layers of stacked aragonite platelets. In recent years, however, GIA laboratories in Bangkok, Hong Kong, Mumbai, and New York have reported unusual pearls with a “nacreous-looking” surface caused by the presence of calcite instead of aragonite (figure 1) (Winter 2022 Lab Notes, pp. 477–478). These findings piqued curiosity and prompted a more comprehensive study involving research samples and mollusk shells. The study indicated that these calcite “nacreous-looking” pearls were likely produced by mollusks belonging to the Placuna genus in the Placunidae family.
These marine bivalve mollusks, commonly known as “windowpane oysters,” are found along the coasts of India, Malaysia, China, and the Philippines (C.M. Yonge, “Form and evolution in the Anomiacea (Mollusca: Bivalvia)-Pododesmus, Anomia, Patro, Enigmonia (Anomiidae): Placunanomia, Placuna (Placunidae, fam. Nov.),” Philosophical Transactions of the Royal Society of London, Vol. 276, No. 950, 1977, pp. 502–503). Placuna mollusks are widely valued for their thin, durable mica-like translucent flat shells, called capiz or kapis shells, which were once a popular alternative to glass and often used as windowpanes and decorations. Although the mollusks were also known to produce small pearls, these were not commercially available until recent decades (C.T. Achuthankutty et al., “Pearls of the windowpane oysters, Placuna placenta,” Mahasagar, Vol. 12, No. 3, 1979, pp. 187–189).
The studied pearls were variously shaped, measuring 3 to 8 mm and weighing 1 to 3 ct. Their bodycolors ranged from silver to brown or gray, with varying degrees of tone and saturation. Viewed under 40× magnification, they exhibited diagnostic surface features of distinctive mosaic or cellular patterns, and some appeared similar to a botryoidal structure. While these features are unique and have not been observed in typical nacreous pearls, a few of the samples also showed overlapping platelets typically found in nacreous pearls (figure 2).
Under long-wave ultraviolet radiation, the pearls displayed a striking reddish fluorescence, particularly from the darker areas, which is comparable to the reactions observed on the windowpane oyster shells (figure 3). Similar reddish fluorescence has been observed in dark pearls from the Pteria species (L. Kiefert et al., “Cultured pearls from the Gulf of California, Mexico,” Spring 2004 G&G, pp. 26–38; Winter 2014 Lab Notes, pp. 295–296). However, it is important to note that the composition of calcite in windowpane pearls and aragonite in Pteria species can be distinguished using Raman spectroscopy.
Ultraviolet/visible (UV-Vis) reflectance and photoluminescence (PL) spectra from a laser excitation at 514 nm collected on the darker areas unveiled features similar to those typically observed in natural dark nacre of Pteria species, as well as Pinctada margaritifera and Pinctada maxima. The UV-Vis spectra exhibited absorption features at 405 and 495 nm. The 405 nm feature is related to uroporphyrin, a type of pigment responsible for gray to black tones in some pearl oyster species (Y. Iwahashi and S. Akamatsu, “Porphyrin pigment in black-lip pearls and its application to pearl identification,” Fisheries Science, Vol. 60, No. 1, 1994, pp. 69–71). All the samples showed bands at approximately 620, 650, and 680 nm in their PL spectra. These three PL bands have been commonly recorded in darker-colored pearls from the species mentioned above.
The saltwater origin was confirmed by energy-dispersive X-ray fluorescence spectrometry, which revealed very low or below detection limit manganese content and high levels of strontium ranging from 1000 to 1500 ppm.
Intriguingly, real-time X-ray microradiography (RTX) uncovered a distinct, evenly spaced fine concentric ring structure throughout most of the inner area of the pearls (figure 4). This structure was identified in most of the submitted pearls as well as the research samples studied. With these combined unique characteristic features, we were able to confidently conclude that the mollusks producing these pearls were from the Placunidae family. The first GIA Pearl Identification report for a pearl from this mollusk was issued in May 2023.
While most shelled mollusks have the potential to produce pearls, identifying the exact species remains a challenge, particularly with less commonly encountered species. Conducting continuous research studies with known samples from reliable sources is crucial and greatly assists in identification. As a continuation of this lab note, a comprehensive article featuring detailed studies will be prepared.
