Pl.: tesserae

A small cube, tag or token. In an architectural context, the term is used to denote the small cubes used to compose mosaics. The tesserae were often made of stone, sometimes of glasspaste or terracotta.

Tesserae in different contexts.
L: loose glasspaste tesserae. The yellow/greenish ones have changed colour after exposure to fire (Joanbanjo, Wikimedia Commons; CC BY-SA 3.0).
R: black and white stone tesserae used to create a figurative scene in the mosaic technique, here on the floor of the vestibule of the Neptune Baths of Ostia (Klaus Heese, Ostia-antica.org).