Raman spectroscopy of natural and cultured pearls and pearl producing mollusc shells

Abstract

The attractiveness of natural and cultured pearls is related, in part, to their colour. Raman spectroscopy was applied to natural and cultured pearls and the shells from 23 species of pearl producing molluscs, thus covering the majority of the natural and cultured pearls and their host animals found in the market today, in order to better understand their colouration and eventually to be able to identify the host animal. Pigments with a polyenic chain were identified in samples from 18 species. The vast majority were found to be a mixture of partially methylated polyenes, similar to those observed in octocorals. Samples with a similar colour but from different animal species can be sometimes related to a significantly different mixtures of such pigments. Only natural pearls from Pinna nobilis were found to be coloured by carotenoids. Raman spectra of other pigments were also observed on samples from four species, possibly linked with pigments from the porphyrin group.

The Diversity of Linear Conjugated Polyenes and Colours in Nature: Raman Spectroscopy as a Diagnostic Tool

“…Conjugated polyenals containing from 6 to 9 C=C unsaturations were identified from parrots’ feathers [5,6,8,42–45] and conjugated polyenals containing from 6 to 14 unsaturations are found in shells, [7,9,33,46] pearls [11,47,48] produced by molluscs and octocorals (Figure 3). [7,29,30,49–56] …”

“…Unlike their carotenoid analogues, psittacofulvins are biosynthesized by animals and they are found keratinised within the feather follicles [43,67] . Intriguingly, molecules with similar spectral features have also been identified by Raman spectroscopy in marine and fresh water invertebrates inserted into calcareous matrices (CaCO 3 polymorphs) of shells, [7,9,32,46] pearls, [11,47,48] and corals [7,29,30,49,51–54,56] . In these organisms, the conjugated length of the double bonds ranges from 6 to 14 unsaturations.…”

“…Studies performed with shells of the marine bivalve Arctica islandica indicated that conjugated polyenals are incorporated within the aragonitic crystals and they are located both on the outside and inside of the shell, developing the same layered pattern typical for the growth edge [78] . Shells and pearls from several molluscs are coloured by molecules such as these, but exceptions were found to occur in the shell of Cypraea moneta , [79,80] recent and extinct (fossil) brachiopod species [33] and in the pearls of Pinna nobilis , [48] where carotenes were identified. In general, carotenoids obtained from the diet are constituents of the lipid bilayer of diverse molluscan cell membranes and are present in some organs (gill, mantle, nervous adductor, leg muscle and hepatopancreas) [81] .…”

“…Resonance Raman spectroscopy (RRS) has been widely used as a method to investigate carotenoid molecules and other conjugated polyenes [3–5,7,11,30,47–56] . Resonance Raman scattering occurs when the wavelength of the laser's excitation is in close proximity to an intense electronic absorption band and this effect selectively enhance Raman signal of these polyenic systems.…”

GeoRaman

“…The Raman identification of pigments (polyenic chains and porphyrins) is also at the centre of the work of Karampelas et al, [33] associating the different pigments with pearls produced by different families of molluscs.…”