“Atypical Beads”: Variations of Two Types of Nuclei
Atypical bead cultured pearls (aBCP) are occasionally encountered during laboratory testing. The nuclei used can take the form of various undrilled, partially drilled, or even drilled materials including natural pearls (abalone, scallop, turban species, Pteria species, and Pinna species), freshwater non-bead cultured pearls, coral, plastic, small shells, faceted sapphire beads of various colors, glass, quartz, and agate (“Atypical ‘beading’ in the production of cultured pearls from Australian Pinctada maxima,” GIA Research News, February 13, 2017). Four aBCPs (figure 1) were recently discovered in a group of 50 loose pearls submitted to GIA’s Mumbai laboratory.
Externally, the four white to light cream-colored aBCPs looked similar to the other pearls submitted in the lot. When viewed under 40× magnification, their surfaces exhibited a typical nacreous surface of overlapping aragonite platelets. Energy-dispersive X-ray fluorescence spectrometry on all four revealed manganese levels between 13.30 ppm and 45.60 ppm and strontium levels between 1064 ppm and 1822 ppm, characteristic of a saltwater environment. Interestingly, optical X-ray fluorescence (XRF) of pearl 1 revealed a strong yellowish green reaction, while pearls 2, 3, and 4 were inert. The ultraviolet/visible reflectance spectra collected on the four pearls showed weak absorption features at around 320–420 nm. Raman analysis using 514 nm laser excitation showed the expected doublet at 702/705 cm–1 and peak at 1085 cm–1, indicative of aragonite.
Real-time microradiography (RTX) and X-ray computed microtomography (μ-CT) analysis revealed a variety of internal structures that required interpretation. RTX imaging of pearl 1 showed a thin irregular demarcation close to the surface which was not continuous. On viewing the μ-CT, the demarcation was more discernible and followed the irregular outline of the pearl (figure 2, row 1). An irregularly shaped freshwater shell bead was clearly used as the nucleus, hence the strong yellowish green reaction observed in the optical X-ray fluorescence unit (figure 3, left). Under transmitted light, banding within the bead nucleus was also observed (figure 3, right); externally, the pearl had notable indentations on its surface.
Pearl 2 hosted the most interesting nucleus of the four. A rectangular “tissue-box” shaped bead, likely a piece of cut saltwater shell given the almost identical radio-opacity to that of its host, was evident in the RTX and μ-CT images (figure 2, row 2) (Fall 2022 Gem News International, pp. 378–380). Externally, the pearl was smooth, lustrous, and free of any surface blemishes.
The RTX and μ-CT images of pearl 3 revealed an obvious demarcation with a linear structure at the center of the nucleus and a small “organic tail-like feature” at one end within the boundary separating the bead from the overgrown cultured nacre (figure 2, row 3). All features were consistent with a saltwater non-bead cultured pearl being used as the bead nuclei. The lack of growth arcs around the demarcation was possibly due to rapid nacre deposition during the culturing process (“Atypical ‘beading’ in the production of cultured pearls from Australian Pinctada maxima,” GIA Research News, February 13, 2017). This pearl exhibited distinct surface scratches that were visible without magnification.
Pearl 4 also showed a strong demarcation feature on the RTX and μ-CT images. However, the central area differed from the other three pearls, as it showed a void-like feature in the middle (figure 2, row 4). The bead nucleus used in the process was most likely a saltwater non-bead cultured pearl based on the fact that the pearl was inert to XRF, in keeping with pearl 3, despite the relatively thin nacre layers that would allow a freshwater bead to react. The pearl was also unusual because it possessed three nearly flat “bases” with concave features at the center of each. This raised questions about its identity even before X-ray examination, since the authors have rarely observed natural pearls with such features.
This is not the first time GIA has encountered aBCPs, but to receive four with variations on two types of nuclei—atypical shell (not typical round shell beads) and non-bead cultured pearls—in one lot was very interesting. This proves that such pearls are still circulating in the market and are being mixed with natural goods in an attempt at deception (Fall 2011 Lab Notes, pp. 229–230). Given the spectral data collected (S. Karampelas, “Spectral characteristics of natural-color saltwater cultured pearls from Pinctada maxima,” Fall 2012 G&G, pp. 193–197) and the pearls’ internal structures and external appearance (A. Homkrajae et al., “Internal structures of known Pinctada maxima pearls: Cultured pearls from operated marine mollusks,” Fall 2021 G&G, pp. 186–205), it is apparent that all four of them formed within Pinctada species mollusks, most likely Pinctada maxima. Atypical bead cultured pearls have always been an interesting and sometimes challenging subject. With modern equipment and practical pearl testing experience, laboratories such as GIA aim to remain one step ahead of the possible experiments used by cultivators.
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