INTERNAL STRUCTURES OF KNOWN PINCTADA MAXIMA PEARLS: CULTURED PEARLS FROM OPERATED MARINE MOLLUSKS

Abstract

Saltwater non-bead cultured pearls, sometimes referred to as “keshi,” are byproducts of the bead cultured pearl cultivation process and are mainly composed of solid nacre, in keeping with natural pearls. They often form as individual pearls or may be seen as additional features on the surfaces of some bead cultured (BC) pearls, sometimes known as “Tokki pearls.” The 86 saltwater cultured samples in this study, comprising 74 non-bead cultured (NBC) pearls and 12 BC pearls with intriguing additional surface features, were collected directly from the gonads of operated Pinctada maxima mollusks by two of the authors during a visit to a pearl farm. They were collected in order to study the internal structures using real-time microradiography and X-ray computed microtomography analyses. Three structural types-including organic-rich concentric, void, and linear-were observed in the NBC samples, as expected for saltwater NBC pearls produced by Pinctada species. The structures were also observed within the associated additional features related to the bead nuclei in the BC pearl samples. The internal features of most of the NBC samples permitted them to be separated from their natural counterparts. Nevertheless, a limited number of NBC samples contained particular structural forms similar to those observed in known P. maxima natural pearls previously studied, or they contained complex overlapping features that do not conform to the majority of structures observed in NBC pearls in this study and previous publications. This illustrates the challenges GIA and other gemological laboratories face with some pearl identification cases. Therefore, GIA's procedure in addressing such cases is discussed and demonstrated. Since the pearl identification stated on reports is an opinion based on a reference collection database and the gemologist's experience, possible differences of opinion among various gemologists and organizations are hardly surprising. This study on known cultured pearls provides additional reliable data that is essential in consolidating GIA's reference collection database on the internal structures of pearls.