Atypical “Box Bead” Cultured Pearls
Gemologists are aware of two main types of bead cultured pearls—bead cultured (BC) and atypical bead cultured (aBC). BC pearls are cultured with the typical round, predominantly freshwater shell bead nuclei, while aBC pearls are cultured with any material that is not typical. Atypical bead nuclei include fancy-shaped shell beads, pearls of any kind, plastic beads, gemstones, or other materials of various shapes. GIA’s Bangkok laboratory received five loose undrilled pearls as research samples for examination. These were reportedly sold as “tissue box pearls” in the Indian market by a trader known to some GIA staff. Externally they exhibited a smooth surface with luster resembling that of South Sea Pinctada maxima cultured pearls. The samples ranged from 8.00 × 7.26 × 6.05 mm to 9.93 × 7.86 × 5.86 mm, weighing between 2.50 and 3.49 ct, and exhibited a baroque form with a white to cream color (figure 1).
Microradiographic examination revealed unusual box-shaped demarcation outlines uncharacteristic of traditional BC pearls (figure 2). To the author’s knowledge, this kind of internal rectangular shape is never seen in natural pearls. Sample 2 revealed an additional structural feature extending from the shorter side of the “box bead” (see red arrow in figure 2). Sample 3 possessed two distinct radio-translucent areas around the box-shaped outline; these are expected for voids or certain organic-rich features in pearls. Samples 4 and 5 also showed similar radio-translucent features, but to a lesser extent. Since the rectangular internal structures are not observed in natural pearls, the clear demarcation features with a box-shaped outline are enough to identify these as aBC pearls.
The box-shaped beads clearly influenced the external shapes of their pearl hosts when they formed within the mollusks, as seen in figure 1. Samples 2 and 3 were cut in half for further examination of their internal structures. Under magnification, each half showed an obvious demarcation between the box-shaped bead and the nacre overgrowth. The “box beads” appeared to be more translucent than traditional beads and exhibited platy structure and banding in some areas, proving they were fashioned from shell. Additionally, the feature in sample 2 possessed a radial structure and appeared somewhat translucent. Observation through a gemological microscope revealed that the gaps around the nucleus in sample 3 were in fact voids with areas of dark organic-rich material, likely conchiolin, present within (figure 3).
Another point of interest was the inert reaction of the shell nuclei in each half of both samples under optical X-ray fluorescence. Since most freshwater bead nuclei show strong yellowish green or greenish yellow reactions, it was clear that the “box beads” were not fashioned from freshwater mussel shells.
Energy dispersive X-ray fluorescence (EDXRF) results of the “box bead” nuclei, outer nacre layers, and the radial calcitic area revealed major amounts of calcium, along with high levels of strontium ranging from 1209 to 1986 ppm, and manganese below 63 ppm. The low levels of manganese detected by EDXRF corresponded to the inert optical X-ray fluorescence reaction (figure 4), as the intensity of any reaction is directly correlated to the manganese levels (P. Kessrapong et al., “Atypical bead cultured Pinctada maxima pearls nucleated with freshwater non-bead cultured pearls,” GIA Research News, April 6, 2020, https://www.gia.edu/gia-news-research/atypical-bcp-nucleated-with-nbcp). The high strontium and low manganese concentrations indicated saltwater origin.
Under long-wave ultraviolet radiation, the sawn surfaces exhibited a bluish reaction. Sample 2 showed additional very weak orangy reactions associated with the columnar feature partially surrounding the bead (figure 5).
Short-wave UV fluorescence spectra were also collected from the pearl surfaces using a GIA-developed spectrometer. The spectra showed two distinct peaks around 330 and 360 nm with counts above 10,000, indicating the surfaces were not processed using bleach or any whitening/brightening agents (C. Zhou et al., “Detection of color treatment and optical brightening in Chinese freshwater ‘Edison’ pearls,” Summer 2021 G&G, pp. 124–133).
Raman analysis using 514 nm laser excitation on the “box bead” surfaces and outer nacreous layers of the sawn surfaces showed typical aragonite features at around 701/704 cm–1 (doublet) and 1085 cm–1. The columnar radiating area on sample 2 was also analyzed, revealing features at around 154, 281, 711, and 1085 cm–1 that were consistent with calcite (J. Urmos et al., “Characterization of some biogenic carbonates with Raman spectroscopy,” American Mineralogist, Vol. 76, 1991, pp. 641–646).
All the data obtained from the advanced testing methods as well as observation under magnification prove the “box bead” nuclei were fashioned from saltwater shell.
Although this is not the first time GIA has encountered saltwater cultured pearls containing non-traditional bead nuclei, the box shape of the beads in this sample group has never been documented in the gemological literature and is therefore notable.