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Forty-two mosaic coloured/opaque "glass" tesserae from three sites (Milan, Italy; Durrës, Albania; Hierapolis, Turkey) situated in the Western and Eastern parts of the Roman/Byzantine Empire, dated between the 5 th and the 9 th centuries, were studied by optical microscopy, SEM-EDX and Raman microspectroscopy in order to investigate the nature of their pigments and opacifiers as well as the microstructure of glass ceramic materials. The Raman signatures of glass matrix and phases dispersed in the soda-lime glassy matrix showed the presence of six opacifiers/pigments. The use of soda ash glass in the tesserae from Durrës (post 8 th c.) allows refining the mosaic debated chronology. The use of soda ash matrix glass together with the presence of calcium antimonates (Ca 2 Sb 2 O 7 and CaSb 2 O 6 ), pyrochlore solid solution/Naples' yellow (PbSb 2-x-y Sn x M y O 7-δ ) and cuprite (Cu 2 O) or metallic copper (Cu°) in many samples show the technological continuity in a Roman tradition. However, the presence of cassiterite (SnO 2 ) and quartz (SiO 2 ) in one sample from the beginning of the 5 th century, diverging from Roman technology, offers a chronological marker to identify newly (not re-used) produced tesserae. Graphical abstract HighlightsOpaque/coloured glass mosaic tesserae exhibit a glass ceramics microstructure.The innovative use of cassiterite (SnO 2 ) and quartz (SiO 2 ) in 5 th century tesserae is evidenced.The technological innovations went alongside the continual use of Roman recipes (calcium antimonate and yellow pigments). NoveltyThe first use of cassiterite and quartz in the beginning of the 5 th c. as well as the use of calcium antimonates after Roman times in the tesserae produced ex novo with mixed glasses were demonstrated.