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 5th and the 9th 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 8th c.) allows refining the mosaic debated chronology. The use of soda ash matrix glass together with the presence of calcium antimonates (Ca2Sb2O7 and CaSb2O6), pyrochlore solid solution/Naples’ yellow (PbSb2−x−ySnxMyO7−δ) and cuprite (Cu2O) or metallic copper (Cu0) in many samples show the technological continuity in a Roman tradition. However, the presence of cassiterite (SnO2) and quartz (SiO2) in one sample from the beginning of the 5th century, diverging from Roman technology, offers a chronological marker to identify newly (not re-used) produced tesserae.