Raman spectroscopy provides structural data useful for opal classification. Because it is sensitive to short-range order and the material water content, this non-destructive method offers more information than the classical classification of Jones and Segnit (1971), based on X-ray diffraction. The position of the apparent maximum of the main Raman band at low wavenumbers can be used to classify opals on the basis of their degree of crystallinity. The most amorphous opals (those from Australia in our study) have a maximum beyond 400 cm (super -1) , the best crystallized ones (those from Mexico in our study) around 325 cm (super -1) . Brazilian opals occupy an intermediate position. Two preliminary results need to be verified on a much larger sampling including all types of known opal deposits: the position of the various bands of Raman spectrum seems to be characteristic of the geographical origin of the sample (at the scale of the geological province), and also of the type of geological origin (volcanic versus sedimentary).

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