A natural protein-related biopolymer film and its positive role in pearl brightening

Abstract To date, the external micromorphological differences among high-luster pearls and the precise relationship between the organic matrix covered on the external surface of pearl and its luster remain, to a large extent, unknown. Here, using scanning electron microscopy combined with energy-dispersive x-ray spectroscopy, infrared spectroscopy, and Raman spectroscopy, we comparatively investigated the multiscale surface characteristics of representative seawater-cultured pearls with different luster grades. The results showed that a natural protein-related biopolymer film of nano-thickness existed on the surface of high-luster pearls. Moreover, the biopolymer film and the microscale tablets self-assembled by aragonite nanoparticles, which serve as a nanoscale organic film and a microscale plane mirror, respectively, together play key roles in producing the excellent luster. In addition, the higher the content of organic matter covering the external surface of pearls, or the thicker the average thickness of the film, the smoother the surface and the better the luster of pearls. The findings substantially enhance our knowledge of the gemological and morphological characteristics of pearls and their corresponding natural brightening mechanism. Most importantly, this study not only provides some key guidelines on the daily maintenance of pearls and the optimization process for pearl brightening but also inspires optical functional material syntheses.