Natural Pearls from Wild Marine Mollusks - Internal Structures - Study by GIA

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.

INTRODUCTION
Pearls are biogenic gem materials that may form naturally without human intervention, or with assistance from humans in a culturing process. Natural and cultured pearls often display similar external appearances and occasionally cannot be differentiated without examining their internal structures. Over the last century, scientists and gemological laboratories have used film X-radiography and digital real-time microradiography (RTX) to reveal these interior growth patterns (Alexander, 1941; Webster, 1950; Benson, 1951; Sturman, 2009; Scarratt and Karampelas, 2020). Around 2010, X-ray computed microtomography (μ-CT) began to be applied to pearl testing. This application provides high-resolution 3D imaging of the morphological structures, allowing fine growth features to be viewed in greater detail compared to traditional X-radiography (Karampelas et al., 2010; Krzemnicki et al., 2010; Otter et al., 2014; Karampelas et al., 2017). RTX and μ-CT are the main techniques used today by GIA and other gemological laboratories for pearl identification.

As with most research, interpretation of the results is based on data collected over the years, as well as the experience of those performing the work. Thus, a sample’s source or the way in which it was obtained are very important factors to consider when creating a reliable database (Pardieu and Rakotosaona, 2012; Vertriest et al., 2019). In accordance with its existing guidelines for collecting gemstone reference samples in the field, GIA has applied pearl sample classification codes reflecting the different degrees of origin dependability (table 1). Those listed as A-type samples are the most dependable, while E-type samples are the least dependable. Collecting pearls directly from freshly opened mollusks is the ideal situation (A and B type), but this is not always possible, especially when it comes to natural pearls. In many cases, research can only be carried out on samples purchased or loaned from pearl farmers or reputable dealers (C and D type). These samples are often described as being “reportedly” from a source or mollusk, and in most cases they serve as useful references in pearl identification matters. However, there are occasions where these reported samples are not sufficiently dependable to reach confident determinations, and more reliable reference samples are needed. E samples lack specific origin information, but can still be useful for some research such as color treatment or surface quality enhancement comparisons. A reference collection constructed of “known samples” that carefully documents how, when, and where they were acquired is an essential foundation for research on origin identification. Samples with known origin provide the highest degree of data reliability.


Read entire article here: https://www.gia.edu/gems-gemology/spring-2021-internal-structures-of-pinctada-maxima-pearls-from-wild-marine-mollusks?fbclid=IwAR2pF9oEAOg6xNLQSvFF256g-SwF0V0wFmgUna4TSK4XTozFDX9RCWditJQ