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Colouration and Genetic Attribute of Chocolate Pearl

QI Li-jian1,HUANG Yi-lan2,C.G.Zeng3(1.Tongji University,Shanghai 200092,China;2.Journal of Gems & Gemmology,China University of Geosciences,Wuhan 430074,China;3.Nan Yang Gemological Institute,Singapore 228208,Singapore)  
The preliminary researches on the chemical compositions,Raman spectrum,reflectance spectrum,surface micro-characteristics and the genetic attribute of the chocolate pearl samples that appear in the market recently are tested by means of LA-ICP-MS,Raman Spectrometer,Micro-optical fibre and other testing instruments.The analysing and testing results show that most of the chocolate pearl samples carry the simple concentric chromatape structure with light-coloured outer layer(brownish yellow,brownish red),dark-coloured inner layer(grayish black) and the uneven coloured brownish red spots and grey spots are distributed randomly along the surface of pearl layer.After the bleaching treatment at the low temperature,the chemical compositions,Raman spectrum and UV-NIS reflectance spectrum characteristics of the chocolate pearl samples change from time to time.The chemical compositions of the chocolate pearl samples are characterized by rich Cu and Pb while lack of K.The red shift of Raman spectrum at 1 276 cm-1 is close to 1 371 cm-1 while the blue shift of Raman spectrum at 1 583 cm-1 is close to 1 545 cm-1and Raman spectrum at 663 cm-1 vanishes.It is obvious that the absorption strength of band c(697 nm) is weakened abruptly,red shift of band a to the long wave and combined with band b to form a wide absorption spectral band within the range of the optical wavelength.It is proved that there is no direct internal relation between the colour of the chocolate pearl samples and the conjugate double-bond electron transition of the melanin,mainly due to the thermal variation effect of the K pigment protein existing in the brownish black Tahitian pearl.
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