Nature of Pigments in Orange and Purple Coloured Chinese Freshwater Cultured Pearls: Insights from Experimental Raman Spectroscopy and DFT Calculations

Pearls, a well-known organic gemstone, are popular for their attractive lustre and rich colour. The pigmentation and colour of pearls have never been clearly explained. Understanding the pigments and colour origin of pearls can be a guide for artificial cultivation and rational conservation. In this study, Chinese freshwater cultured pearls were collected as research samples. The appearance and colour characteristics of pearls were characterised using D65 standard light source photography and UV–Vis spectroscopy, the molecular structure of the pigments in the pearls was characterised using Raman spectroscopy, and Density Functional Theory (DFT) calculations were used to reveal the characteristics of the pigments in the pearls in terms of molecular structure and electronic excitation. It was proposed that freshwater pearls are coloured with polyene pigments, with the chain length of the polyene determining the type of colour and the concentration of the polyene determining the colour intensity of the pearl. The HOMO–LUMO transition of conjugated polyenes is intrinsically responsible for the colour of pearls. Many colour-rich biominerals also have similar Raman spectral features to pearls, and this study has wider implications for understanding the nature of pigments and their colour origins.