Abstract
Some cultured and natural pearls can be reliably distinguished by visual inspection and by the use of lens and microscope. However, assessing the origin of the pearls could be not straightforward since many different production techniques can now be found in the pearl market, for example in salt or freshwater environments, with or without a rigid nucleus. This wide range of products requires the use of new effective scientific techniques. Indeed, X-ray radiography has been used by gemologists since last century as the only safe and non-destructive way to visually inspect the interior of a pearl, and recently, also X-ray computed micro-tomography was used to better visualize the inner parts of the gems. In this study we analyzed samples of natural and cultured pearls by means of two non-destructive techniques: the X-ray Phase-Contrast Imaging (PCI) and the Neutron Imaging (NI). PCI and NI results will be combined for the first time, to better visualize the pearls internal morphology, thus giving relevant indications on the pearl formation process.
Highlights
Natural pearls were considered precious and rare gems for many centuries
Two non-destructive techniques, the X-ray Phase-Contrast Imaging (PCI) - performed at ID17 biomedical beamline, European Synchrotron Radiation Facility (ESRF) - and the Neutron Imaging (NI) - performed at ICON beamline, Paul Scherrer Institute (PSI), were applied to the study of very complex structures like the pearl specimens
The resolution of few microns gives the possibility to visualize small fissures in the inner part of the pearl (Fig. 3d), which are usually due to the dehydration or the ageing of the pearl. Both tomographic reconstructions allow to identify not just the nacre composed of aragonite, and aragonite structures in the core of the pearl that cannot be disclosed with neutron or X-ray radiography
Summary
Natural pearls were considered precious and rare gems for many centuries. There is an important price gap between the two kinds of pearls: cultured pearls may cost a few hundreds of dollars while the natural pearls can be sold for millions of dollars. The calcifying matrix is a mixture of proteins, glycoproteins and polysaccharides that precisely self-assemble and control the CaCO3 polymorph, the size, the shapes of the crystals and their orientation[6] The complexity of these structures is proved by the fact that not all the biochemical reactions involved during the pearl genesis are understood yet. The analysis of pearls was mainly based on the X-ray radiography[7,8] that allows to visualize slight variations in X-ray absorption This technique is often inefficient to reveal small intrinsic structure variations of pearls. Two non-destructive techniques, the X-ray Phase-Contrast Imaging (PCI) - performed at ID17 biomedical beamline, European Synchrotron Radiation Facility (ESRF) - and the Neutron Imaging (NI) - performed at ICON beamline, Paul Scherrer Institute (PSI), were applied to the study of very complex structures like the pearl specimens. Species and variety Environmental Shape Measurements Weight Drilling Body colour Overtone Luster Surface quality
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