Saltwater Clamshell Beads (Tridacna Species) Used in Freshwater Cultured Pearls
GIA’s Bangkok laboratory recently studied 23 intriguing freshwater bead cultured pearls purchased from a vendor in Hong Kong. They displayed an elongated baroque shape with a surface protuberance at one end, and they weighed approximately 20 ct apiece and ranged from 16.73 × 12.58 × 12.49 mm to 18.98 × 13.53 × 13.43 mm (figure 1). Their shape and appearance are similar to saltwater bead cultured pearls known in the market as “Tokki pearls” (A. Homkrajae et al., “Internal structures of known Pinctada maxima pearls: Cultured pearls from operated marine mollusks,” Fall 2021 G&G, pp. 186–205).
Real-time microradiography (RTX) revealed the distinct round demarcation of a bead nucleus with additional void and/or linear features in all the samples, as expected from their external appearance (figure 2, left). However, the beads were more radiopaque and appeared lighter gray in RTX imaging than the traditional freshwater shell bead nuclei widely used in both saltwater and freshwater bead cultured pearl production (P. Southgate and J. Lucas, The Pearl Oyster, 2008, p. 286; “Freshwater pearling in Tennessee,” GIA Research News, October 7, 2016; N. Sturman et al., “Vietnam: Shell nuclei, pearl hatcheries, and pearl farming,” Fall 2020 G&G, pp. 402–415). Numerous long, thick parasite tubes were visible within the beads, something not often seen in traditional beads (figure 2, right). Optical X-ray fluorescence imaging was used to verify the pearls’ freshwater origin. They exhibited weak to moderate yellowish green fluorescence, confirming a freshwater origin. However, the reaction was much weaker than many freshwater bead cultured pearls of the same size and nacre thickness previously examined. These unusual characteristics observed in RTX and X-ray fluorescence imaging led us to cut two pearls in half to investigate the bead nuclei.
The cross sections revealed bead nuclei approximately 12 mm in diameter. The beads appeared chalky white and opaque, unlike traditional beads that are commonly white to cream-colored and semitranslucent. Moreover, the beads exhibited a porcelain-like texture and a subtle flame pattern. We ground the surface of two additional samples to better understand the material. Banded structures of shell formation with subtle flame structures in between were present (figure 3).
Energy-dispersive X-ray fluorescence was used to analyze the elemental concentration of the outer nacre layer and the bead nuclei. The nacre layer showed a high level of manganese and low strontium content, confirming the pearls formed in a freshwater environment. Conversely, the bead nuclei showed high strontium (around 1000 ppm) and no manganese was detected, which are typical results for pearls from a saltwater environment. X-ray fluorescence imaging of the cross section displayed a strong yellowish green reaction on the freshwater nacre due to high manganese content, while the saltwater bead nuclei were inert (figure 4). A Raman spectrometer with 514 nm argon-ion laser excitation was employed to analyze the bead nuclei composition, and double peaks at 701 and 705 cm–1 and a single peak at 1085 cm–1 of aragonite were recorded. Aragonite is a common component of shell and pearl.
The large size, white color, porcelain surface appearance, banded structure with flames, saltwater formation environment, and aragonite composition of the bead nuclei suggested they were created from the clamshell of Tridacna-species mollusks. Fashioned shells from Tridacna species have been applied particularly in pearl imitations (see Summer 2014 Lab Notes, pp. 153–154; Spring 2015 Lab Notes, pp. 62–63) but are not commonly used in culturing because they are easily broken during the drilling process. More importantly, shells from Tridacna species should be avoided for jewelry items or decoration since they are protected by the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES), the international agreement that protects endangered species.
