The art of creating mosaic patterns grew and developed with the rise of the Byzantine Empire in the 5th century. New characteristics appeared in mosaics with Eastern influences in style and the use of glass tesserae, known as smalti, sourced from northern Italy. This added new texture and life to the mosaic patterns being created, with the smalti, which were cut from thick sheets of colored glass and had a rough surface and tiny air bubbles throughout, being backed by reflective silver or gold leaf.
The application of mosaics also changed; while the Romans favored the use of mosaic patterns for flooring, the Byzantines took the art further and applied them to walls and ceilings. They kept their smalti un-grouted to allow light to reflect and refract through them and set their pieces at slight angles to capture the play of light as it moved through the space and allow the silver or gold backing to sparkle from every angle.
The mosaic patterns and themes melded together, with Roman images being absorbed into the predominantly Christian themes favored by the Byzantines, and while some of the pieces remained purely decorative, some were used to depict Emperors much as the Romans depicted gods (Source).