Partially Hollow Tridacna Blister Pearls with Shells Attached
GIA sees pearls of all types submitted to its global laboratories. Almost all of them are either loose or mounted in jewelry pieces; however, exceptions are occasionally encountered. The submission of a blister pearl or blister still attached to its shell is such an example (Winter 2015 Lab Notes, pp. 432–434). In January 2017, the Bangkok laboratory received an intact shell with a pearl attached near the adductor muscle area (figure 1, top). The item weighed 2.36 kg. The shell measured 32.0 × 21.0 × 11.5 cm, while the pearl measured 52.0 × 47.5 mm. At the same time and by sheer coincidence, another shell with a similar appearance and a pearl attached to the same area (figure 1, bottom) was submitted to the New York laboratory. This item weighed 806.40 g and the shell measured 20.0 × 13.5 × 9.5 cm, while the pearl measured 80.0 × 50.0 mm.
The exterior of the shell submitted to Bangkok exhibited a light brownish color and appeared roughly triangular in outline with a wavy pattern of thin ridges in rows, while the interior was white to cream with a porcelain-like surface. According to the client, the shell was found in 2014 by fishermen off the coast of Kood Island, a district of Trat Province in eastern Thailand. The shell’s features are characteristic of Tridacna species mollusks, of which there are a number of varieties (U.E. Hernawan, “Taxonomy of Indonesian giant clams (Cardiidae, Tridacninae),” Biodiversitas, Vol. 13, No. 3, 2012, pp. 118–123).
As figure 1 (top right) shows, a blister pearl of similar color is prominently attached to the surface. Observation with a loupe and microscope confirmed that it was naturally attached and untreated. Microscopic examination using a fiber-optic light source confirmed the presence of flame structure on some surface areas of the shell and pearl, confirming their non-nacreous or porcelaneous nature. The pearl’s flame structure was short and patchy, while that seen on the shell was sharper and more defined. While the pearl’s nomenclature may be the source of some debate, we considered it to be a blister pearl, rather than a blister, based on its external appearance, position on the shell, and size (E. Strack, Pearls, Ruhle-Diebener-Verlag, Stuttgart, Germany, 2006, pp. 115–127). The shell and pearl were exposed to long-wave UV, with the shell showing a moderate to strong chalky blue reaction with yellowish orange patches near the lip area (figure 2, left), while the blister pearl exhibited a weak to moderate yellowish green color (figure 2, right). This demonstrates how fluorescence in samples may vary from area to area.
However, the most noteworthy feature of the Bangkok specimen was that when the shell was tilted or gently rocked from side to side, a liquid clearly moved within the blister pearl (figure 3, left). The liquid was not viscous and so was most likely water, rather than a thicker liquid such as oil. It is possible that seawater was trapped during the blister pearl’s formation or found its way into the “hollow” pearl at a later date. When fiber-optic light was used to illuminate the blister pearl, some of the light was transmitted and made the pearl appear translucent (figure 3, right). This, together with the trapped liquid, proved the pearl was at least partially hollow (see video at http://www.gia.edu/gems-gemology/tridacna-blister-pearls).
To see the extent of the Bangkok blister pearl’s void, we examined its internal structure using a real-time X-ray (RTX) machine. Although it was only possible to examine the pearl in one orientation owing to its position and size, the blister pearl quickly revealed its partially hollow form (figure 4, left). The various shades of gray within the white near-oval feature (solid pearl surface) prove that the blister pearl’s interior is full of organic matter and/or air since the X-rays passed through with very little obstruction. While hollow or partially hollow pearls—both nacreous and non-nacreous—are not new (N. Sturman, “Pearls with unpleasant odors,” GIA Laboratory, Bangkok, 2009, www.giathai.net/pdf/Pearls_with_unpleasant_odours.pdf), this is the first one GIA has encountered with such visible trapped liquid. Although the solid shell attached to the pearl displayed similar gray shades, this coloring relates more to the sample’s thickness than anything else. A few small rounded, darker gray patches are voids caused by parasites boring within the shell rather than by structures within the blister pearl.
Void-like structures in whole or blister pearls from Tridacna species mollusks are not unusual (S. Singbamroong et al., “Microradiographic structures of natural non-nacreous pearls reportedly from Tridacna (clam) species,” Proceedings of the 5th GIT International Gem and Jewelry Conference, Pattaya, Thailand, pp. 200–222), and GIA has examined many voids in other non-nacreous pearls. The RTX results proved that the specimen described was a natural blister pearl attached to its shell.
The shell submitted to GIA’s New York laboratory is also noteworthy, not only for the coincidental submission, but also because the pearl was larger relative to its host’s size than the one examined in Bangkok. The baroque natural blister pearl attached to this rather more colorful shell was also partially hollow, although not to the degree of the Bangkok sample. As the RTX results in figure 4 (right) revealed, the partially hollow blister pearl had a complex internal structure and was less homogenous than the specimen submitted in Bangkok. Unfortunately, no provenance was supplied with the New York sample, so there is no record of where it was found.
Both GIA reports stated that the naturally attached feature on each shell was a natural blister pearl. Many such specimens examined at GIA’s labs are submitted as loose examples that have already been removed from their hosts, so it was a welcome change to handle these two shells. In addition, a comment was included on each report informing the clients of the pearl’s partially hollow nature and, in the case of the Bangkok submission, the presence of a liquid. The size and appearance of the shells submitted proved they were not Tridacna gigas (giant clam), and the report referred to the hosts as Tridacna species only.