Fine Linear Structures in Non-Bead Cultured Pearls
Saltwater non-bead cultured (NBC) pearls are defined by the World Jewellery Confederation (CIBJO) as those without an internal bead nucleus that have “formed accidentally or intentionally by human intervention in marine pearl oysters.” These are commonly referred to as “keshi” pearls in the trade. GIA’s Mumbai laboratory has received many pearl submissions that revealed internal structures typical of NBC pearls such as large voids, elongated linear structures, and some with loose concentric “gaps.” However, a recent submission of 23 loose pearls for pearl sorting service proved quite challenging to identify. Externally, all of them looked similar, with a white to light cream color and a medium to high luster. Some possessed a few surface blemishes, but on the whole they were quite clean and appeared cultured.
A majority of the pearls in the lot exhibited concave bases, while a few showed bases with concentric, graduating white to cream-colored rings, with distinct central white frosty patches which were part of the inner subsurface nacreous layers. Four pearls revealing different forms of linear structures were selected from this lot. All were white and button shaped, with sizes ranging from 3.24 × 2.55 mm to 10.33 × 6.84 mm (see above). Externally, all their surfaces exhibited typical nacreous overlapping aragonite platelets, with a fingerprint-like pattern when viewed under 40× magnification.
Energy-dispersive X-ray fluorescence spectrometry on the four pearls revealed low manganese levels ranging between 12.60 and 41.10 ppm and high strontium levels between 1767 and 2883 ppm. Additionally, optical X-ray fluorescence analysis yielded an inert reaction, thus confirming their saltwater origin. The pearls showed a moderate bluish green reaction when viewed under long-wave ultraviolet radiation and a weaker bluish green reaction under short-wave ultraviolet radiation. Raman analysis using 514 nm laser excitation showed a doublet at 702 and 705 cm–1 and peaks at 1085 and 1463 cm–1 indicative of aragonite.
Real-time microradiography (RTX) and X-ray computed microtomography (μ-CT) analysis revealed internal structures that were not straightforward and required more work to reach conclusive results. RTX imaging of pearl 1 showed a prominent, very fine central linear feature horizontal to the base (table 1, row 1). Analysis with μ-CT revealed a more obvious linear feature showing that it was unaligned with the other growth arcs that generally followed the shape of the pearl. This structure was observed in most of the samples in the submitted lot. Externally, a small white frosty central spot was visible on the concave base surrounded by graduating circular rings.
RTX images of pearl 2 revealed a small circular light gray core associated with undefined gray arcs curving along the core in a distorted pattern (table 1, row 2). Moreover, only a couple of weak growth arcs were observed in the outer nacre layers. The structure was very similar to those observed in some natural pearls with small dense cores, making it inconclusive (A. Homkrajae et al., “Internal structures of known Pinctada maxima pearls: Cultured pearls from operated marine mollusks,” Fall 2021 G&G, pp. 186–205). Further μ-CT analysis helped define the main feature, a short and fine linear structure accompanied by a minute void-like feature.
RTX images for pearl 3 revealed a medium-size button-shaped central organic-rich core, which was radiotranslucent and had a thin gray outline (table 1, row 3). This was surrounded by numerous fine growth arcs that followed the shape of the core. Although this organic-rich core looked quite natural from the RTX images, μ-CT images revealed a short linear feature in the center of the core, suggesting an NBC pearl.
The largest sample, pearl 4, had been partially drilled, and the drill hole was quite large (table 1, row 4). It was unlike the thinner and shorter drill holes often used for weight retention in natural pearls. While the choice of drill hole thickness is not decisive evidence on its own, it may be a useful indicator (Spring 1986 Lab Notes, pp. 51–52; Summer 1986 Lab Notes, p. 111). Externally, the pearl had a rounded base and a circled circumference with multiple deep indentations associated with the circled grooves. The RTX images showed minute metal remnants (visible as radiopaque white specks) within the drill hole and extensive curved growth arcs throughout the pearl, with additional arcs located on adjacent boundaries. The pearl showed minimal tight growth structure, and its external appearance aroused suspicion. Further examination via μ-CT analysis revealed a small linear feature that had been partially removed by the drilling. Reaching a conclusion would be very challenging, if not impossible, using RTX analysis alone.
Given the volume of button-shaped pearls the Mumbai laboratory has been testing, it is quite evident that large quantities of such pearls are circulating in the market. These pearls mostly have smooth surfaces and high untreated luster. The linear features could not be identified in all the pearls using RTX alone due to a number of factors including size, appearance, radiotranslucency, and orientation. It is not difficult to remove minute linear structures by drilling, and such pearls could easily be passed off as natural due to the presence of fine growth features around the center. But with the availability of advanced technology, especially μ-CT imaging, remnants of the linear structure were evident in certain orientations. In some cases, with the proper alignment of the drill hole, such small linear structures may also be visible in the RTX images. This requires considerable skill and patience to align, starting from the thickest direction first with respect to the shape of the pearl and then repositioning the same pearl 90° in the profile view. Therefore, it may be risky to rely on RTX results alone (S. Karampelas et al., “X-ray computed microtomography applied to pearls: Methodology, advantages, and limitations,” Summer 2010 G&G, pp. 122–127).
By analyzing these challenging internal structures using modern equipment, GIA and other laboratories aim to protect consumers by creating awareness of them and minimizing the chances of misidentification.
