Coloring Factors of Natural and Dyed Golden Pearls and Research Progress on Their Identification Methods

Color-causing components of natural golden pearls are mainly metal ions, porphyrin and carotenoids. Organic dyes are the color-causing components of dyed pearls. Conventional observation, UV-vis Absorption Spectroscopy, Raman Spectroscopy, and Fluorescence Spectroscopy are used to distinguish natural from dyed pearls. The identification methods of the two types of pearls (natural golden and artificially dyed golden) are reviewed and presented in this study. Conventional observation is necessary because the color of dyed pearls is not homogenous and dye can be concentrated in the hole or any damaged surface. UV-vis Absorption Spectroscopy is a commonly used identification method. The main absorption peaks of natural golden pearls lie between 350 nm and 360 nm, whereas those of dyed pearls lie between 410 nm and 450 nm. Raman Spectroscopy has identified that natural golden pearl has a characteristic peak of 275 cm^-1, but dyed pearl has a strong fluorescence background peak. Fluorescence Spectroscopy has identified that the natural golden pearl has a central peak of 472 nm as excitation by 376 nm light, whereas the dyed pearl has a central peak of 436 nm as excitation by 372 nm light. Further verification of these identification methods should be undertaken, along with improvements in optimization and the development of new methods.