A tessera (plural: tesserae, diminutive tessella) is an individual tile, usually formed in the shape of a square, used in creating a mosaic. It is also known as an abaciscus or abaculus.
The unique illuminating effect of golden tesserae, Cappella Palatina in Palermo, Sicily, 12th centuryA mosaic made from tesserae in St Peter's Basilica, Vatican State, ItalyA close up of the bottom left corner of the picture above. Close inspection of the full picture reveals the individual tesserae.