Raman Spectroscopy in Corals and Pearls

Introduction: Corals and pearls are organic gem material used for ornamental purposes. Apart from black and gold corals, formed by hornlike organic matter, red to pink corals and pearls consist mainly of calcium carbonate in calcite or aragonite forms. The formation and nature of calcium carbonate have been largely investigated by means of Raman spectroscopy [1], [2], [3]. Raman scattering may be helpful to distinguish between coral and pearl species through the identification of their calcite or aragonite structure, or differences in the pigments. Experimental: Here we report on the Raman spectra taken at 632.8 nm and at 473.1 nm with a microRaman Labram HORIBA Jobin-Yvon instrument equipped with 50x objective on a series of red to pink corals and on natural pearls. Results: The Raman spectrum of coral identifies the biogenic calcium carbonate phase of the skeleton. The Raman spectrum of natural-color coral typically reveals additional peaks related to organic pigments. Pearls show the aragonite peaks at 206 cm -1 , 702-706 cm -1 and at 1086 cm -1 .