Non-Destructive Identification of Gray Akoya Pearl and High-Quality Irradiated Gray Freshwater Pearl

Recently, some high-quality irradiated gray freshwater pearls have appeared in the market, which are difficult to distinguish from Akoya gray pearls by naked eyes. In order to distinguish them, 9 grey Akoya pearls, 11 grey freshwater pearls irradiated by γ-ray and 2 freshwater pearls without irradiation treatment were collected. The colour, surface morphology, trace elements, and spectral characteristics were characterized by gemstone microscopy observation, scanning electron microscopy observation, UV fluorescence, X-ray fluorescence spectroscopy, UV-Visible spectroscopy and Raman spectroscopy, respectively. Four identification evidences were obtained: (1) grey Akoya pearls have a rich orient, with regular and continuous contour lines, while irradiated grey freshwater pearls have a simple orient, with irregular contour lines. Under long wave ultraviolet light, gray Akoya pearls show strong blue white fluorescence and the irradiated gray freshwater pearls show moderate blue white fluorescence; (2) gray Akoya pearls are rich in Sr and poor in Mn, while irradiated gray freshwater pearls are the opposite; (3) In the visible light range, irradiated gray freshwater pearls exhibit absorption peaks around 400 nm, while gray Akoya pearls have absorption bands ranging from 455 nm to 550 nm; (4) Raman spectra showed characteristic peaks of freshwater pearls at 1 125 cm^-1 and 1 508 cm^-1, as well as 1 131 cm^-1 and 1 523 cm^-1 in purple-toned and pink-toned irradiated gray freshwater pearls, respectively, and gray Akoya pearls did not exhibit those 2 sets of characteristic peaks.