Study on Pigments in Freshwater Pearls and Hyriopsis Cumingii Based on Raman Spectra and EEM Spectra

Colour is an important factor in evaluating the quality of pearl and directly affects its value. But definitive conclusion about the pigments of pearls has not yet been determined. During the growth process, the colour of pearl may be affected by its mussel. In order to study the relationship between pigments in pearls and mussels, Hyriopsis cumingii and its non-nucleated pearls were selected and their Raman spectra and excitation-emission matrix (EEM) spectra were collected. The theoretical Raman spectra of possible pigment structures were calculated by quantum chemistry. The results show that the Raman spectral characteristics of pearls of different colours are similar but also different. The similarity is that they all present the characteristics of polyene pigments: they all have the stretching vibration of C=C double bond and the stretching vibration of C-C single bond. While the Raman peak positions of pearls with different colour perform different. This is due to the different conjugated chain lengths of polyene pigments. The polyene pigments in orange yellow pearls and purple pearls contain 10 C=C double bonds and 12 C=C double bonds, respectively. In terms of EEM spectral characteristics, the darker the pearl colour, the weaker the fluorescence intensity. The fluorescence centers are located at 284 nm/340 nm. The Raman spectra and EEM spectra of freshwater pearls and Hyriopsis cumingii shells are almost identical, indicating a high degree of similarity in their composition. It is speculated that the secretion and metabolism of pigments in the biomineralization process of the two are very similar.