Mosaic glasses are very heterogeneous materials, and their characterization highly benefits from investigation by micro-analytical techniques. In this regard, ancient mosaic tesserae belonging to Daphni Monastery (Greece, XI century) were analysed by micro-Raman spectroscopy. In all the samples, some silicon nanocrystals were disclosed within the silica amorphous matrix of glass mosaics. This result, together with statistical analysis, suggests that a singular glass production site was the origin of all the investigated glass tesserae supporting the hypothesis of local glass making in contrast with raw materials supplied only from Middle East or Egypt. Copyright © 2012 John Wiley & Sons, Ltd.