Internal structures of known pinctada maxima pearls: Natural pearls from wild marine mollusks

Abstract

Natural pearls form in mollusks without any human assistance, whereas cultured pearls form as a result of human intervention. In general practice, the identification of natural versus cultured pearls is determined by the internal structure revealed by X-ray techniques, particularly real-time microradiography (RTX) and X-ray computed microtomography (μ-CT). Interpretation of the results is based on reference samples studied previously. Therefore, a reference database founded on reliable samples obtained directly from known sources is an important factor. The internal structures of 774 natural marine pearls collected in situ by two of the authors from freshly opened wild Pinctada maxima mollusks were studied in detail to gain a better understanding of the internal structural characteristics of natural P. maxima pearls. Based on the internal features obtained from RTX and μ-CT analyses, the samples were classified into six broad growth structure types: (1) tight or minimal growth, (2) organic-rich concentric, (3) dense core, (4) void, (5) linear, and (6) miscellaneous structures. Tight or minimal growth structures are typically observed in natural pearls and was noted in the majority of these samples. Some exhibited particular forms of organic-rich concentric, void, or linear structures resembling those previously observed and reported in some non-bead cultured (NBC) pearls produced from the same mollusk species. Such overlapping features demonstrate the challenges of distinguishing some natural and NBC pearls submitted to gemological laboratories. This article will present the diverse range of internal features found in natural P. maxima pearls, discuss the complexities sometimes encountered during the identification process, and share the protocols GIA applies in such situations. The work strengthens GIA's reference collection database on the internal structures of pearls, supporting its goal of providing dependable results on pearls submitted by clients. In this context, the authors intend to conduct further studies on natural and cultured pearls of known origins from various environments and mollusks to make GIA's database even more comprehensive.