Limitation on Ultraviolet-Visible Spectrometer to Distinguish Golden Pearl and Dyed Golden Pearl

With the popular of golden pearl ornaments,more and more dyed golden pearls appear on the market.Since the price of natural colour golden pearls and dyed golden pearls is extremely different,the colour detection is quite important.Gemmologists and detection experts try to analyze the differences between golden pearls and dyed golden pearls with the help of Raman spectrometer,ultraviolet-visible spectrometer(UV-Vis),laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS),fluorescence spectrometer and cathodo luminescence(CL).UV-Vis spectrometer is considered to be one of the effective ways to identify the colour of golden pearls,but this method is not mature and has some controversial opinions with testing results because of the relatively short time development.The previous studies considered that the UV-Vis spectrum peaks between 350~360nm were caused by natural pigment of golden pearls and obvious peaks between 410~450nm were caused by the organic dye of dyed golden pearls.However,in practical work,the testing results of UV-Vis spectrometer are quite different in different parts of golden pearls.Sometimes it shows conflicting results between microscope observation and UV-Vis spectra,since some golden pearls being natural colour also have absorption peaks at 400nm.Limitation of UV-Vis spectrometer on studying the cause of golden colour of the pearls is discussed and it is concluded that UV-Vis spectrum absorption peaks at 400nm are related to the structure of pearl layer.