tessera
    n 1: a small square tile of stone or glass used in making
         mosaics

Tessera \Tes"se*ra\, n.; pl. Tesserae. [L., a square piece, a
   die. See Tessellar.]
   A small piece of marble, glass, earthenware, or the like,
   having a square, or nearly square, face, used by the ancients
   for mosaic, as for making pavements, for ornamenting walls,
   and like purposes; also, a similar piece of ivory, bone,
   wood, etc., used as a ticket of admission to theaters, or as
   a certificate for successful gladiators, and as a token for
   various other purposes. --Fairholt.
   [1913 Webster]