Colouration and Genetic Attribute of Chocolate Pearl

QI LI-jian 1 , HUANG Yi-lan 2 , C.G.Zeng 3

Tongke Gemmological Institut e, Tongji Universit y,Shanghai 200092,China;
2. Journal of Gems & Gemmolog y,China University of Geosciences,Wuhan 43004,China
3.Nan Yang Gemmological Institut e,Singapore 228208, Singapore

Abstract: The preliminary researches on the chemical compositions Raman spectrumreflectance spectrum surface micro-characteristics and the genetic attributes of the chocolate pearl samples that appear in the market recently by means of LA-ICP-MSRaman Spectrometer , Micro-optical fibre and other testing instruments. The analysing and testing results show that most of the chocolate pearls samples carry the simple concentric chromatape structure with light-colored outer layer (brownish yellow, brownish red) , dark-colored inner layer (grayish black) and the uneven colored 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 composition, Raman Spectrum and UV-NIS reflectance spectrum characteristics of the chocolate pearl 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 1276cm-1 is close to 1371 cm-1while the blue shift of Raman spectrum at 1583cm-1 is close to 1545 cm-1 and Raman spectrum at 663cm-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 within the range of the optical wavelenght. It is proved that there is no direct internal relation between the color 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.

Keywords: chocolate pearl,K pigment protein, colored mechanism.

*Published in Journal of Gems and Gemmology, 2008, Vol 10, No.1, page 20-24.