Gem Corals - X-ray diffraction, solid state NMR, elemental analysis

The crystallographic and chemical features of twenty-one (21) gem-quality corals belonging to both the classes Hydrozoa and Anthozoa of the phylum Cnidaria have been investigated by powder X-ray diffraction (XRD), ¹³C Magic Angle Spinning (MAS), NMR and laser ablation inductively-coupled plasma mass spectrometry (LA-ICP-MS). The carbonatic matrices of the skeletons of these corals were found to be calcite (in seven samples) or aragonite (in 14 samples). The content of Ca is higher in aragonitic corals than in the calcitic corals. The values of Sr/Ca and Mg/Ca ratios indicate that the incorporation of strontium or of magnesium in the coral skeleton depends on the host carbonate polymorph. These ratios generally have good correlation with the variation of the cell volumes, so that Mg and Sr content can be considered the best discriminating chemical feature between these corals. The trace element (Li, B, Ti, V, Cr, Co, Ni, Zn, Rb, Y, Zr, Nb, Cs, Ba, Pb, U) content displayed broad variations that often overlapped both in calcitic and aragonitic corals.