Early Roman Empire intaglios from 'rescue excavations' in Paris: An application of the Raman microprobe to the non-destructive characterization of archaeological objects

Three coloured, polished and beautifully engraved small stone objects (intaglios) were recently discovered during 'rescue excavations' in central Paris which uncovered relies of Early Roman Empire roads and buildings. Mineral identification was of course desirable, but without extracting any material for analysis. This presented an ideal application of the Raman microprobe (RMP), a non-destructive technique which can be used to characterize materials without sample preparation. The RMP spectra obtained directly from the untouched objects unequivocally identified all three intaglios as chalcedony, which is the general name for microcrystalline alpha-quartz. The colours of the samples indicate the variety names which can be applied. This study also detected the presence of a zircon microinclusion and of the recently recognized new mineral species moganite (which is another polymorph of SiO2) intermixed with the alpha-quartz. This raises important questions relating to (1) the relative stability ranges of alpha-quartz and of moganite and (2) the terminology of many varieties of microcrystalline alpha-quartz, which in fact are not composed of alpha-quartz alone but an admixture of alpha-quartz and moganite. (C) 1997 by John Wiley & Sons, Ltd.