Among organic gems, pearls are widely used for decorating artworks and precious jewels; due to their attributes of beauty, they are also in high demand in the nowadays gemstone trade. The nature of color in pearls is still debated, even if recent studies have attributed to mixtures of methylated (carotenoids) or non-methylated polyenes with different UV-VIS absorption spectra and chain lengths [1]. Previous studies have demonstrated the potential of Raman spectroscopy in detecting pigments in cultured freshwater pearls [2]. With the aim to improve knowledge on the nature of pigments in natural and cultured pearls, several pearls species (Japanese cultured saltwater, Chinese natural and dyed freshwater, oriental natural, Tahitian cultured pearls) have been investigated by Raman spectroscopy using several equipment (laboratory set-up vs. portable) and excitation sources (632.8, 532, 473.1 nm). Colored natural species showed characteristic Raman double peak indicating pigment belonging to the carotenoid family, with C-C (ν2) and C=C (ν1) stretching modes at 1159 cm-1 and at 1516 cm-1. Overtones and combination modes are observed for all species up to > 4000 cm-1 at 473 nm, in resonance conditions. The ν2 and ν1 frequencies depend on the polyene chain length, i.e. on the number N of the conjugated double bonds. The Raman spectra of pinkish pearls may be interpreted on the basis of a mixture of polyene pigments with different chain lengths. A linear relationship between ν1 and ν2 and a simple approximate rule for their dependence on N: ν1 = 254*103/(N+157); ν2 = 189*103/(N+157) has been proposed [1]. On this basis, contributions from N=6 to N=11 may be found by peak deconvolution. Polyenes have been identified as responsible of color in freshwater and saltwater pinkish and yellowish pearls; finally, in some Chinese freshwater pearls Raman spectra suggested the use of synthetic dyes. Aknowledgements The authors are very grateful to the gemmologist Ugo Longobardo for having provided the studied pearls. References [1] L. Bergamonti, D. Bersani, D. Csermely, P.P. Lottici, Spectroscopy Letters 2011, 44, 453. [2] S. Karampelas, E. Fritsch, J.Y. Mevellec, J.P. Gauthier, S. Sklavounos, T. Soldatos, J. Raman Spectrosc. 2007, 38, 217.
Nature of pigments in natural and cultured pearls by using Raman spectroscopy
BERSANI, DANILO;Simona, Raneri
;
2017-01-01
Abstract
Among organic gems, pearls are widely used for decorating artworks and precious jewels; due to their attributes of beauty, they are also in high demand in the nowadays gemstone trade. The nature of color in pearls is still debated, even if recent studies have attributed to mixtures of methylated (carotenoids) or non-methylated polyenes with different UV-VIS absorption spectra and chain lengths [1]. Previous studies have demonstrated the potential of Raman spectroscopy in detecting pigments in cultured freshwater pearls [2]. With the aim to improve knowledge on the nature of pigments in natural and cultured pearls, several pearls species (Japanese cultured saltwater, Chinese natural and dyed freshwater, oriental natural, Tahitian cultured pearls) have been investigated by Raman spectroscopy using several equipment (laboratory set-up vs. portable) and excitation sources (632.8, 532, 473.1 nm). Colored natural species showed characteristic Raman double peak indicating pigment belonging to the carotenoid family, with C-C (ν2) and C=C (ν1) stretching modes at 1159 cm-1 and at 1516 cm-1. Overtones and combination modes are observed for all species up to > 4000 cm-1 at 473 nm, in resonance conditions. The ν2 and ν1 frequencies depend on the polyene chain length, i.e. on the number N of the conjugated double bonds. The Raman spectra of pinkish pearls may be interpreted on the basis of a mixture of polyene pigments with different chain lengths. A linear relationship between ν1 and ν2 and a simple approximate rule for their dependence on N: ν1 = 254*103/(N+157); ν2 = 189*103/(N+157) has been proposed [1]. On this basis, contributions from N=6 to N=11 may be found by peak deconvolution. Polyenes have been identified as responsible of color in freshwater and saltwater pinkish and yellowish pearls; finally, in some Chinese freshwater pearls Raman spectra suggested the use of synthetic dyes. Aknowledgements The authors are very grateful to the gemmologist Ugo Longobardo for having provided the studied pearls. References [1] L. Bergamonti, D. Bersani, D. Csermely, P.P. Lottici, Spectroscopy Letters 2011, 44, 453. [2] S. Karampelas, E. Fritsch, J.Y. Mevellec, J.P. Gauthier, S. Sklavounos, T. Soldatos, J. Raman Spectrosc. 2007, 38, 217.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
