Conch pearl is a calcareous concretion produced by the Queen conch mollusk and is of extremely high economic value. Its attractive pink color has remained a mystery for hundreds of years. This difficulty limits the development of cultivation and preservation technology of conch pearl. Due to its super-high value, it is generally impossible to use destructive chemical analysis methods. Here, various nondestructive spectroscopic analysis methods and theoretical calculations were used to reveal the color origin of pink conch pearl. The experimental Raman and UV-Vis spectroscopies were used to characterize the color and pigments. We further discuss the pigments in conch pearl by comparing the Raman spectral characteristics and pigments of the red coral. Based on the characteristics of the experimental spectra, we speculate that the carotenoids color the conch pearl. The theoretical Raman and UV-Vis spectra of all-trans polyene and carotenoids were simulated using DFT calculations. According to the results, we proposed that the pigments in pink conch pearl are polyenes containing 11 shortened C–C bonds with multiplicities > 1.5, which may be demethylated carotenoids.
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.
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