Compositional Characterization and Color Genesis of Precious Coral Based on Multi-Spectroscopic Techniques

The color origin of precious coral, a highly valued biogenic polycrystalline gemstone, has long remained elusive. In this study, an integrated approach employing spectrophotometry, Raman, FTIR, and UV-Vis spectroscopy, coupled with Spearman correlation analysis, was utilized to investigate a color-graded series of precious coral samples ranging from white to red. The results demonstrate that the calcareous composition of the samples tested in our study consists exclusively of calcite. The actual chromophores are identified as a blend of multiple distinct polyene species, characterized by Raman shifts at 1126 and 1515 cm⁻¹, with density functional theory (DFT) calculations determining the number of conjugated (C=C) bonds in the polyene chain to be 10–11. Inherently exhibiting a red-orange hue, the progressive accumulation of these polyenes drives a systematic color transition from orange to red. Both absorption bands at 314 nm and 532 nm in the UV-Vis spectra are attributed to the polyene pigment molecules. Specifically, the broad 532 nm band is dominated by π-π* electronic transitions, while the 314 nm band likely arises from terminal benzene rings and their derivatives. As the pigment concentration increases, this band exhibits pronounced broadening and an increase in absorbance, accompanied by a redshift in the maximum absorption peak. This spectral evolution leads to an intensified absorption in the yellow-orange region, elucidating the intrinsic mechanism underlying the color transition of precious coral from orange to red with increasing pigment content. This work lays a solid foundation for the non-destructive identification of precious corals and future research on their color genesis.