A multimodal microcharacterization of trace-elements in defective pearls by SEM-CL, EPMA, µ-XRF and CONFOCAL RAMAN-IN-SEM imaging.

Pearls are greatly valued by the jewelry industry as shiny organic gems. Their glossiness, results of a defense mechanism against foreign particles. Pearls (Hyriopsis cumingii) are produced from a natural biomineralization process controlled by organic molecules. Pearl biocrystal is a hybrid composite: organic / mineral (3 % / 97 %). The regular form of pearl mineralization is aragonite, an orthorhombic polymorph of calcium carbonate. Each biocarbonate plateletis a polygranular composite of aragonite nanograinsstuck by proteins. It diffracts as a single crystal and is so called mesocrystal. Platelets have a polygonal shape, few hundreds of nanometersthick, whichstack along the c-axis with chitin organic 'cement'. This layered structure produces the well-known 'pearly' aspect. Sometimes, pearls show a biomineralization defect characterized by a lack of shine (also called 'milky pearl'). It has been established that this defect is related to the change in crystallization form: from orthorhombic aragonite to hexagonal vaterite [1]. Fig. 1: SEM observation of the interface: same area as seen with cathodoluminescence (CL, left) and in secondary electron mode (SE, right)