Porphyrins And Metal Porphyrin Pearl Color And Color-causing Mechanism

The article focuses on the coloring mechanism of pearls, due to their organiccomponents namely porphyrin and carotene, by ICP-AES, Florescence, UV-VisSpectrascopy, SEM and Micro-total chemical analysis system . 134 sample pearlsare studied, including white, yellow, pink and purple freshwater pearls, gray and blackseawater pearls and breached pearls.The coloring of pearls includes two contributions: internal coloring and externalcoloring. The former further includes the body color of the mother oyster, the color ofthe nucleus and mantle pieces seeded, the trace elements in the water environment,the temperature, PH and density of the water. The latter, the essential contribution andthe focus of this article, includes organic pigments (porphyrin, carotene), metalelements and organic articles.The experiment with ICP-AES shows that there are many types of trace elementsin pearls. The elements in Pearls are mostly the same in variety, but may causedifferent colors of pearls for their different densities. There are bands near 660nm inthe fluorescence spectra of the 7 pearl samples and around 420nm and 500-600nm inthe UV-Vis Spectra, which respectively are the typical Soret absorption band and Qabsorption band of porphyrin. All these prove the presence of the organicpigment—porthyrin. Meanwhile, with the ICP-AES analysis on the organic samplessubtracted from pearls, the article suggests that pearls are colorized by porphyrin andmetal porphyrin. In the freshwater pearls series, white color is produced by Mg-andZn-porphyrin;pink color is caused by Mg-and Fe-porphyrin;yellow color is theresult of Cu-and Zn-porphyrin;purple color comes from Fe-and Zn-porphyrin. In theseawater pearls series, gray and black colors result from Fe-and Mn-porphyrin.Furthermore, according to the theory of organic chromophore, it is thought that thed-π chromophore and π–electron chromophore in the structure of porphyrins cause thecolor of pearls.Therefore, the article concludes: the color of pearls can be changed by means oftransferring the porphyrin properly, which gives rise to a new way in the treatmentand enhancement of pearls.