The Byzantine architectural decoration technique that uses small colored cubes set into mortar is mosaic.
Byzantine mosaics were assembled from small pieces called tesserae. Glass, colored stone, marble, and sometimes gold-backed glass were cut into cubes and pressed into a layer of mortar. The angle and surface of each tessera could be adjusted to catch light, producing a shimmering effect in domes, apses, walls, and vaults.
Gold-ground mosaics became especially associated with Byzantine churches. The reflective background did not represent ordinary space; it created an otherworldly setting for Christ, the Virgin Mary, saints, emperors, and biblical scenes. Mosaics could also communicate political relationships, as imperial figures were shown presenting gifts to Christ or the church.
Mosaic should not be confused with fresco, which applies pigment to wet plaster. Byzantine churches often used both techniques, but mosaic was particularly durable and visually powerful in humid or ceremonial interiors. Major examples survive in Constantinople, Ravenna, Thessaloniki, Sicily, and other former Byzantine regions.