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

The color origin of precious coral, a highly valued organic 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 skeleton 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. 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 originate from the polyene pigment molecules. Specifically, the broad 532 nm band is dominated by π-π* electronic transitions. As the pigment concentration increases, this band exhibits pronounced broadening and enhancement, accompanied by a redshift of 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.