Large Tridacna Shell with a Blister Pearl and a Shell Blister
A collection of various shells was recently submitted to the Mumbai laboratory. Among these was a large shell from the Tridacna species, featuring an attached blister pearl and a shell blister (figure 1). The shell was sawn on three sides to create a stable base for display and weighed approximately 1.37 kg.
Externally, the off-white shell exhibited a wavy pattern of thin, sharp ridges in rows, along with several parasitic holes. Unlike shells from other mollusks, Tridacna shells exhibit thick growth layers of calcium carbonate in a rippling, overlapping pattern. In the thick cross section of this shell, the growth layers opened into a large cavity at one end, divided into two parts by a thin, wavy layer of calcium carbonate (figure 2A).
The interior surface was white to light cream in color, with a smooth porcelain-like surface featuring two protuberances that appeared to be a blister pearl and a shell blister, each naturally attached and displaying distinct striations toward their respective outer edges. The light gray blister pearl with brownish patches on its body, measuring approximately 37.31 × 25.42 mm, displayed a relatively smooth porcelaneous surface with a short and patchy flame-like structure (figure 2B). The dull cream-colored shell blister, measuring 28.93 × 16.63 mm, protruded outward and resembled a cone, with a concave opening at its apex encircled by a brown area rich in organic matter (figure 2C).
Under long-wave ultraviolet radiation, the shell and shell blister exhibited a moderate to strong chalky blue reaction, while the blister pearl displayed reddish purple patches on its surface. Energy-dispersive X-ray fluorescence analysis could not be conducted due to the size of the specimen. Raman analysis of both the blister pearl and the shell blister showed a weak doublet at 701 and 704 cm–1 and a peak at 1086 cm–1, indicative of aragonite, with no indication of color treatment.
The identity of both the blister pearl and the shell blister was confirmed by real-time X-ray microradiography (RTX) analysis. Both features exhibited cavity or void-like structures that have previously been observed in whole pearls and blister pearls from the Tridacna species (Summer 2017 Lab Notes, pp. 231–233). The blister pearl revealed a globule-like structure at its base, with a dark void of varying opacity and organic-rich material (figure 3, left). Its firm attachment to the shell was visible at one end, with discernible shades of gray indicating organic material, confirming its identity as a blister pearl. Conversely, the shell blister displayed a faint void lacking distinct features and contained light gray tubular features indicative of the parasitic boreholes and channels that contributed to its formation (figure 3, right). Parasite channels in the shell were displayed as dark gray lines in random patterns and different thicknesses.
Studies have shown that blister pearls develop as whole pearls within the mantle, normally appearing as tall domes fused with the shell, while shell blisters are caused by foreign bodies and appear as shallow bumps on the shell’s inner surface (“Natural shell blisters and blister pearls: What’s the difference?” GIA Research News, August 26, 2019). Differentiating between blister pearls and shell blisters can be challenging as they can have a similar surface appearance; thus, identification must be confirmed using internal growth structures. The presence of both within the submitted shell provided gemologists with valuable insights on their unique appearance and formation, offering significant gemological and scientific information for future reference.
