Blister Pearl vs. Shell Blister from Pteria sterna
The saltwater mollusk Pteria sterna, also known as the rainbow-lipped pearl oyster, is widely distributed in the eastern Pacific, from Baja California to Peru. These mollusks can grow up to 14 cm, producing natural pearls ranging from 3 to 6 mm and rarely up to 11 mm (CIBJO Pearl Guide, 2020). Externally, their shells are more concave than those of Pinctada-species mollusks, and the internal nacre exhibits vibrant multicolored hues with a metallic sheen. This characteristic is reflected in the wide range of colored pearls they produce.
GIA’s Mumbai laboratory recently received a variety of shells for scientific examination. The submission included three shells from Pteria sterna, exhibiting nacre colors ranging from brown to gray, with vivid overtones of violet and blue and a strong orient with variations in saturation and tone. The shells weighed 63, 83, and 174 g; each had a prominent protuberance attached to its inner wall, measuring approximately 20.22 × 13.78 × 13.55 mm, 23.49 × 19.06 × 15.62 mm, and 24.88 × 17.03 × 12.23 mm, respectively (figure 1).
Upon examination, dried organic remnants from the aquatic habitat and various parasite intrusion boreholes of differing thickness were observed on all three. Shell 2 in particular had a noticeable cavity in the center of its exterior, which extended into the interior area of the protuberance. A distinct feature observed in all three samples was an irregular, patchy nacreous yellowish brown area, indicative of an adductor muscle scar (figure 2). Viewed under high magnification, the nacreous area revealed spiral patterns of aragonite platelets on the surface (figure 3), similar to those observed in most nacreous pearls from Pteria-species mollusks (L. Kiefert et al., “Cultured pearls from the Gulf of California, Mexico,” Spring 2004 G&G, pp. 26–39).
Energy-dispersive X-ray fluorescence (EDXRF) spectrometry of samples 1 and 3 showed manganese levels of 26 and 110 ppm, and higher strontium levels of 1185 and 1915 ppm, respectively, indicative of their saltwater origin. Due to instrument limitations and the position of sample 2, it was not possible to collect EDXRF data from it. All three samples also exhibited a strong reddish reaction under long-wave ultraviolet light (figure 2) and a weak yellowish reaction under short-wave ultraviolet light. The reddish reaction is linked to a type of porphyrin pigment found in pearls originating from the Pteria species (Winter 2014 Lab Notes, pp. 295–296).
Ultraviolet/visible reflectance spectra were collected on multiple spots on the blisters as well as their shells. Absorption bands at identical positions but with different relative intensities were observed at around 405 and 495 nm, indicating a natural color characteristic of the Pteria mollusk. Photoluminescence analysis of the pearls revealed the triple bands at 620, 650, and 680 nm, corresponding to their natural coloration (S. Karampelas, “Spectral characteristics of natural-color saltwater cultured pearls from Pinctada maxima,” Fall 2012 G&G, pp. 193–197).
Real-time X-ray microradiography (RTX) imaging of the samples revealed large voids of varying size and opacity, containing organic-rich material of lower opacity with a spotty and granular appearance. However, the images were not clear due to overlapping dark parasitic network structures beneath the growth within the host shells. All three protuberances within the shells displayed a pronounced boundary around the bulge, making them stand out prominently from their host shell. However, the irregularly shaped protuberance observed in shell 1 stood out due to its well-defined outline at the attachment point, forming a clear separation from the shell beneath it. In contrast, the protuberances from shells 2 and 3 appeared to have grown more together with the shell at their bases, lacking clearly defined outlines at the attachment points. These features are consistent with RTX observations; sample 1 showed a large organic-rich area followed by a more complete nacre growth around the base, while samples 2 and 3 showed a merging of the bases with the shell growth. Based on the characteristics shown in their base attachment, outline shapes, dome height, and internal growth features, it can be concluded that sample 1 is likely a blister pearl, while samples 2 and 3 lean more toward shell blisters.
Shell blisters result from foreign objects trapped in the space between the mantle tissue and shell, which is subsequently overgrown by layers of calcium carbonate, whereas blister pearls are whole pearls that have perforated from the mantle tissue and attached themselves to the shell (E. Strack, Pearls, 2006, Ruhle-Diebener-Verlag GmbH, Stuttgart, pp. 125–127). Previous studies have found that natural shell blisters show some similarities to natural blister pearls in that both have nacreous concretions protruding from the inner shell wall (“Natural shell blisters and blister pearls: What’s the difference?” GIA Research News, August 26, 2019). Differentiating between natural blister pearls and shell blisters remains complex and challenging, relying heavily on external examination and the experience of the gemologist, along with references from observations and research studies. This ongoing learning curve means that even experienced gemologists may reach different conclusions in such cases. GIA is privileged to study such intriguing shell samples, and hopefully these observations will promote a better understanding of natural blister pearls and shell blisters.
