Dynamic microstructural characteristics of Edison pearls cultured in Hyriopsis cumingii

Chinese Edison pearls, invented and produced in China by the Zhejiang Grace Pearl Jewelry Co., Ltd, are usually large and round with variable but attractive colors . Edison pearls have entered the jewelry market in 2011. However, the fine microstructural characteristics of these pearls remain largely unknown, particularly the thickness of the nacre and the spatial distributions of organic pigment in the nacre. Herein, using X-ray microradiography analysis, field-emission scanning electron microscopy, and Raman spectroscopy, we comprehensively investigated the internal fine-microstructural characteristics of typical colored Edison pearls. Our results indicated that Edison pearls have a typical core–shell structure, in which the core is the nucellus and the shell is the nacre. Average nacre thicknesses varied from 1.00 to 3.60 mm among specimens. Surprisingly, theoretical calculations showed that nacre accounted for about 50–90% of the whole pearl by volume. In addition, multiple prismatic or irregular structures, almost entirely composed of aragonite, coexisted in the nacre of some Edison pearls; one of these structures was closely adjacent to the nucleus. Interestingly, the average thickness of individual aragonite tablets in the nacre gradually decreased from the inner nacre (near the nucleus) to the outer nacre (near the pearl surface). Finally, the intensity ratios of the two peaks in the Raman spectra ( I 1124–1134 / I 1085 ) were non-uniform across nacre regions, and generally increased with distance from the nucleus. This pattern was directly attributed to variations in pigment content among nacre regions. Our results substantially increase our understanding of nacre biomineralization and the gemological characteristics of colored Edison pearls. More importantly, this study identified certain characteristics that can be used to identify color-treated Edison pearls.