Spectroscopic Characteristic of Natural and Dyed Yellow Akoya Pearl

In recent years, yellow Akoya pearls have become increasingly popular in the market, but the gemmological characteristics research are relatively scarce. In addition, dyed yellow Akoya pearls have already appeared in the trade as a substitution. In this paper, 24 pieces of natural and dyed Akoya pearl samples were selected and analyzed by X-ray fluorescence spectrometer (XRF), UV-Vis-NIR reflection spectroscopy, and Raman spectroscopy. 2 types of pigments were identified in natural yellow Akoya pearl samples: One is polyene pigment with C-C and C═C as the main structure, and the Raman shift is around 1 130 cm^-1 and 1 530 cm^-1; the other may be a kind of porphyrins, and the Raman shift is around 1 385 cm^-1 and 1 573 cm^-1, and the UV-Vis-NIR spectra show 2 characteristic absorption bands (330-385 nm and 385-430 nm). The spectra of the second pigment type are consistent with golden South Sea pearls. Dyed Akoya pearl samples can be identified by dye concentration on surface pits or along the drill hole. A combination of UV-Vis-NIR spectra and Raman spectra is necessary when surface features are absent. The UV-Vis-NIR spectra of dyed pearls show a weaker absorption at 280 nm, and two absorption bands centered at 330 nm and 460 nm, which are different from the reflectance spectra of natural yellow pearls. The Raman spectra of dyed Akoya pearls lack Raman shifts of natural pigments.