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 relics 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 α-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 α-quartz. This raises important questions relating to (1) the relative stability ranges of α-quartz and of moganite and (2) the terminology of many varieties of microcrystalline α-quartz, which in fact are not composed of α-quartz alone but an admixture of α-quartz and moganite.