Flame-Like Surface Structure in a Nacreous Atypical Bead Cultured Pearl
GIA’s Mumbai laboratory recently received an atypical bead cultured pearl with a unique surface appearance. The white button-shaped pearl weighed 8.15 ct and measured 11.73 × 11.35 × 9.03 mm (figure 1).
Viewed under 40× magnification, the surface exhibited typical fine nacreous overlapping aragonite platelets, but a distinct pattern similar to the flame structures observed in some porcelaneous pearls was also noted (Summer 2021 Lab Notes, pp. 152–153). These flame-like features were formed due to the presence of relatively opaque, frosty white rounded depressions on the pearl’s surface. Although rounded, they seemed to be oriented longitudinally and were more tapered toward the base and apex and broader along the sides of the pearl (figure 2). Also observed on the surface were small white spots with slightly elongated comet tails that flowed tangentially to the circumference of the button. The base and apex of the pearl lacked the whitish subsurface opaque features that produced the flame structure effect, and these two areas appeared more translucent than the other parts of the pearl.
Under long-wave ultraviolet radiation, the pearl showed a strong yellowish green reaction, and a similar but weaker reaction was noted under short-wave ultraviolet radiation. The deep-UV (<230 nm) radiation of the DiamondView instrument revealed a clear bluish reaction, with the surface showing a chalky white flame-like structure extending from the apex center across the pearl. Raman analysis of the pearl’s surface using a 514 nm excitation showed a doublet at 701/704 cm–1 as well as peaks at 1086 and 1464 cm–1 characteristic of aragonite. The photoluminescence spectra revealed high fluorescence, with a maximum centered at approximately 620 nm.
Energy-dispersive X-ray fluorescence spectrometry revealed a low manganese level of 16.9 ppm and a higher strontium level of 1577 ppm, indicative of a saltwater environment. However, when exposed to X-ray fluorescence (XRF) analysis, the pearl exhibited a moderate yellowish green fluorescence with a higher intensity toward the sides but a weaker reaction on the base and apex (figure 3). Saltwater pearls are usually inert when tested by this method, while freshwater pearls and saltwater bead cultured pearls with freshwater shell nuclei tend to fluoresce weak to strong yellowish green. In the case of bead cultured pearls, a thinner nacre usually produces a stronger reaction; conversely, the thicker the nacre, the weaker the reaction.
Real-time microradiography imaging (RTX) revealed a long, complex linear feature in the center typically associated with non-bead cultured pearls (figure 4, left). When viewed under X-ray computed microtomography (μ-CT), the linear feature looked very similar to those often observed in freshwater non-bead cultured pearls. A faint boundary was also seen around the central linear feature (figure 4, right) and other growth rings. All these observations, together with the XRF reaction of the pearl and the saltwater chemistry, were indicative of an atypical bead cultured saltwater pearl with a freshwater pearl used as its nucleus (P. Kessrapong and K. Lawanwong, “Atypical bead cultured Pinctada maxima pearls nucleated with freshwater non-bead cultured pearls,” GIA Research News, April 6, 2020). The nacre thickness overlying the freshwater non-bead cultured pearl nucleus ranged from approximately 1.09 to 1.90 mm. The thicker nacre was positioned near the base and apex and the thinner nacre toward the sides. This aligned with the XRF reaction observed, as a thinner saltwater nacre will allow more fluorescence from the freshwater pearl nucleus to pass through the nacre layers, while thicker nacre masks the reaction.
Over the years, GIA has encountered similar atypical bead cultured pearls with sizeable freshwater non-bead cultured pearls used as a nucleus (Spring 2023 Lab Notes, pp. 74–76). However, the presence of flame-like structures seen on nacreous pearls is a very rare phenomenon. This combination of interesting culturing along with the unique surface features make this pearl noteworthy.
