tes·ser·a 
(tĕsər-ə)
(tĕsər-ə)n. pl. tes·ser·ae (tĕsə-rē′)
One of the small squares of stone or glass used in making mosaic patterns.
[Latin, from Greek, neuter of tesseres, variant of tessares, four; see kwetwer- in the Appendix of Indo-European roots.]
tesserac. 450 bc mosaic mask made with turquoise, coral, and stone tesserae
Teotihuacán, Mexico