Colouration Attributes and UV-NIS Reflection Spectra of Various Golden Seawater Cultured Pearls

In view of the colouration attributes,colour alteration and the identification,the chemical compositions,Raman spectra,reflection spectra,colouration attributes and the surface micro-characteristics of the undyed and dyed golden seawater cultured pearl samples on the recent market are investigated by using the LA-ICP-MS,Raman spectrometer,UV-NIS micro spectrometer and the other testing instruments.The results show that the chemical compositions and the features of the UV-NIS reflection spectra of the golden seawater cultured pearl samples adhere and inherit the basic attributes of the golden-lip shellfish to a large extent.The UV-NIS absorption band centred at 356 nm(±2 nm) can be used as one of the diagnostic evidences in separating undyed from dyed golden seawater cultured pearls.The organic dye used for enhancing colour of the dyed pearl samples show the distinctively different absorption bands from that of the undyed ones.The surfaces of the superficial layer dyed golden seawater cultured pearl samples are of an evenly coloured golden penetration whose thickness is about 15 μm(normally<20 μm).Under the range of the UV visible light,there are two major groups of absorption bands.One is at 427 nm(±2 nm) related to the artificial dying agent,and another is at 353 nm(±2 nm) which belongs to the organic colourants originally present in the pearls.On the other hand,the outer layer of the deep-layer dyed golden seawater cultured pearl samples are of an evenly coloured golden penetration whose thickness is about 50~80 μm with only one diagnostic absorption band at 426 nm(±2 nm) caused by the artificial colouring agent.The features of the absorption bands of the golden inner nacre of the golden-lip shellfish samples are basically the same as that of the undyed golden seawater cultured pearl samples.The relative intensity of the absorption band at 356 nm(±2 nm)is positively correlated to the concentration of the yellow organic pigments present in the nacre.In this paper,the relationship between the colour attributes of the golden seawater cultured pearl samples and the golden-lip shellfish samples is discussed.