Byzantine mosaics
From the 5th century onwards, with the rise of the Byzantine Empire, centered on Byzantium (now Istanbul, Turkey), the art form took on new characteristics. A great impetus to the art occurred when Constantine, in establishing himself on the throne of Byzantium, commenced to give his capital an imperial appearance. These included Eastern influences in style and the use of special glass tesserae called smalti, manufactured in northern Italy. These were made from thick sheets of colored glass. Smalti have a rough surface and contain tiny air bubbles. They are sometimes backed with reflective silver or gold leaf. By the 6th century, as seen in the mosaics in Ravenna, Italy, the technique had changed again. The tesserae now were set in an irregular fashion rather than flat, to catch the light from all possible angles. Silver and gold leaf were crafted into clear glass to enhance this effect. Imagine these walls lit only by candles and lamps. Unlike electricity which provides even lighting, flickering flames cast uneven light in multiple directions, adding to the mystical qualities of the figures.
There are several areas of distinction for Byzantine mosaics, one of which was that where Roman mosaics were mostly used as floors, the Byzantines specialized in covering walls and ceilings. The smalti were ungrouted, allowing light to reflect and refract within the glass. Also, they were set at slight angles to the wall, so that they caught the light in different ways. The gold tesserae sparkle as the viewer moves around within the building. Also, Roman images were absorbed into the typical Christian themes of the Byzantine mosaics, although some work is decorative and some incorporates portraits of Emperors and Empresses.

The mosaic here is from the ceiling of the baptistery in Florence, Italy. Other spectacular examples can be found in Ravenna, Venice and Sicily and in Istanbul.

 
 
Islamic
In the 8th century in the west of Europe, the Moors brought Islamic mosaic and tile art into the Iberian peninsula, while elsewhere in the Muslim world, stone, glass and ceramic were all used in mosaics. In contrast to the figurative representations in Byzantine art, Islamic motifs are mainly geometric and mathematical. Examples can be seen in Spain at the Great Mosque at Cordoba and the Alhambra Palace. In Arabic countries a distinctive decorative style called zillij uses purpose-made ceramic shapes that are further worked by hand to allow them to tessellate (fit together perfectly to cover a surface).