“Lily Pad” Structure in an Orangy Yellow Pearl
GIA’s Mumbai laboratory recently received for identification an orangy yellow semi-baroque pearl weighing 4.14 ct and measuring 9.51 × 8.43 × 7.51 mm (figure 1). Externally, the pearl had a distinct indentation on its top portion and possessed a medium surface luster. Observation of the surface under 40× magnification showed typical fine overlapping platelets of aragonite.
Energy-dispersive X-ray fluorescence revealed a manganese level below detection limits and a strontium level of 1226 ppm. When exposed to X-ray fluorescence, the pearl showed an inert reaction. Both results confirmed the pearl’s saltwater origin. Under long-wave ultraviolet light, it exhibited a yellowish green reaction. Raman analysis using a 514 nm laser excitation revealed a doublet at 701 and 704 cm–1 and a peak at 1085 cm–1, indicating the presence of aragonite. Photoluminescence spectra revealed two strong broad peaks at 620 and 650 nm as well as one weak peak at 680 nm, confirming the pearl’s natural color origin.
Upon further testing using real-time X-ray microradiography (RTX), the pearl displayed an unusual acicular feature resembling the shape of a lily pad, with tiny flame-like structures radiating from the center. This feature was intersected by several growth arcs (figure 2A). X-ray computed microtomography (μ-CT) provided a more detailed view, revealing a distinct small void at the center. Surprisingly, the acicular “lily pad” feature observed in the RTX images appeared as numerous elongated, spiky structures with very fine lines cutting through the shape (figure 2, B and C).
To further study this feature, three-dimensional models were created using specialized software to render the μ-CT scan images (C. Zhou et al., “New 3-D software expands GIA’s pearl identification capabilities,” GIA Research News, May 13, 2016). In both the μ-CT scan images and the 3D models shown in figure 3, it was evident that this unique feature was confined to the central part of the pearl and did not extend to the outer nacre layers. Notably, the lily pad feature had elongated spikes that were thicker at one end and gradually tapered as they radiated outward from the center.
Gemological laboratories do encounter pearls with internal structures that are very difficult to positively identify. Due to the lack of conclusive evidence proving it was a cultured pearl combined with the presence of a rare and distinctive natural structure, this pearl was considered noteworthy. Such intriguing natural growth structures can be influenced by several factors, including environmental conditions, the health of the mollusk during formation, and other external causes impacting the pearl formation process. The precise mechanism of natural pearl formation continues to be widely debated, and encountering such pearl samples causes us to further contemplate the creation of these exquisite gems.
