Inside black pearls

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Highlights

  • Inner structures of pearls are irregular and diverse.

  • Pearls are not reversed shells.

  • Similar structures are known in round or pear shape pearls.

  • Some pearl structures are unknown in the pearl oyster shell.

Abstract

The quality of natural and cultured pearls depends on the size, shape and properties of the surface. The inner structure of the pearl is not known, and not taken into account. Indeed, it is assumed that the cultured pearl is a nucleus covered by a thin organic layer, a thin prismatic layer and a thick nacreous layer. The examination of spherical and pear shape samples resulting from the grafting of the black lip pearl oyster (Pinctada margaritifera) shows that this assumption is, in most cases, erroneous. Optical, electron and atomic force microscopic observations, infrared and Raman analyses, as well as micro X-ray fluorescence and micro X-ray absorption near-edge spectroscopy data show that the inner structure of a pearl does not depend on its shape. Both circle or pear shape and spherical pearls show a large variability of the inner structures and composition. There are no two identical samples. Moreover, the arrangement is irregular within a single pearl, and some structures (aragonitic pseudo-prisms) do not exist in the shell.

Introduction

Pearls, naturally produced by molluscs, have been used for jewellery for centuries. Now man has taken advantage of the ability to create cultured pearls in farms. Both freshwater and marine species are harvested, the large Pinctada being the most common in seawater. For the most common technique for freshwater pearls, two animals are used: a donor from which a small piece of the mantle is cut, and the recipient oyster in which the piece of mantle is inserted. For marine pearls, a nucleus (also called “bead”) obtained from the nacreous layer of the shell of a freshwater bivalve, is inserted with the small piece of mantle. The location of the nucleus and piece of mantle varies. Usually, they are inserted in the gonad. Then, for several months or years, the “pearl layer” secreted by the mantle is deposited on the nucleus. The main part of the cultured pearl is the nucleus, the pearl layer being thin: a minimum of 0.8 mm is recommended for black pearls. Examination of the outer surface does not allow to distinguish between natural and cultured pearls, but sections or X-ray radiographs show the nucleus inserted during the grafting process necessary to produce cultured pearls (Fig. 1a).

The quality of a pearl depends on the shape, size, colour and luster of the nacreous layer, also called pearl layer. All these parameters are usually characterized only from the surface of the samples. Sometimes, the thickness of the pearl layer is known using X-ray radiographs and taken into account. It is hoped that the pearl will be spherical, because of the shape of the nucleus, and because it is believed that the deposit of the pearl layer is a regular process similar to that of the shell secretion. The first mineral layer of the pearl would be prismatic calcite, then the nacreous layer would be the outer layer (Fig. 1b), such as the pearl is similar to a “reverse shell”. Nevertheless, most collected cultured pearls are not spherical. The details of the inner structure of the regular spherical pearls are usually unknown. Nevertheless, sections across irregular and non-spherical pearls, also called baroque or circled pearls, have shown the structure is complex: not only the shape is not regular, but the layered arrangement is disturbed. Such anomalies exist in cultured and non-cultured pearls [[1], [2], [3], [4], [5]] (Fig. S1). More recently, CT-scan images have displayed similar features [[6], [7], [8]]. Despite the fact that it is often said that X-ray tomography is non-destructive, secondary unexpected modifications (colour for example) have been observed on modern samples (Cuif unpublished data, [9]). Thus, such techniques are not appropriate for coloured pearls, such as natural black pearls harvested in French Polynesia.

The pearl layer represents all the secreted material resulting from the graft process. It is often believed that the whole pearl layer is nacre [10,11], but even in spherical pearls, it is not always true [12]. Aragonite takes the form of nacre or “prisms”, and calcitic prisms are present. Few data are available on the variability of the inner structure and mineralogy of black pearls produced by the black lip pearl oyster Pinctada margaritifera. To address this problem, we have studied the inner structure of several dozen cultivated black pearls from French Polynesia. Micro- and nanostructure, mineralogy and composition of the “pearl layer” were studied, combining the following techniques: scanning electron microscopy, atomic force microscopy, X-ray computed tomography, Raman micro-spectroscopy, laboratory and synchrotron-based Fourier transform infrared micro-spectroscopy, synchrotron-based micro X-ray fluorescence and micro X-ray absorption near-edge spectroscopy (XANES) at the sulphur K-edge.

Section snippets

Material

Pearls were produced at the Polynesian “Centre des Métiers de la Nacre et de la Perliculture” at Rangiroa (Tuamotu). All the grafting operations were made by two professional grafters, in order to reduce influence of diversity in grafting techniques. Grafts were prepared from donor oysters belonging to Pinctada margaritifera (the “black lip” pearl oyster), used on all production sites in French Polynesia. All receiver oysters were grown in similar conditions in the Rangiroa lagoon. Pearls were

Results

A baroque pearl with a regular pear shape was selected as a “reference” (sample A). The colour of the outer shiny surface is a dark green (Fig. 2a).

Discussion and conclusion

Pearls are natural gemstones and occur in a wide variety of shapes. Round pearls are by no means the only shape, and perfectly round pearls are actually quite rare. The highest quality pearl is considered to be a round pearl, but pear shape pearls are also expensive when they have a perfect outer surface and are not circled. Despite the fact that the pearl industry as a whole has not adopted a universally used standard grading system, irregular pearls are rejected and not sold.

It is often

Declaration of competing interest

Authors declare that they have no conflict of interest.

Acknowledgments

This research was supported by a SYNTHESYS grant (GB-TAF), ESRF grants (EC208, EC545, LS2371), SOLEIL Grant 201770784. ADEQUA project was funded by the Polynesian administration for Fisheries and Aquaculture. We thank K. Fletcher (ESRF) for improving the English language. We also thank two anonymous reviewers for reading and constructively criticizing the manuscript.

Data availability

The raw/processed data required to reproduce these findings cannot be shared at this time due to legal or ethical reasons.

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