tessera
noun
tes·sera ˈte-sə-rə 
plural tesserae ˈte-sə-ˌrē
-ˌrī
1
: a small tablet (as of wood, bone, or ivory) used by the ancient Romans as a ticket, tally, voucher, or means of identification
2
: a small piece (as of marble, glass, or tile) used in mosaic work
Example Sentences
Recent Examples on the Web On the other hand, VERITAS would provide a map of every tessera, albeit with less overall detail. 
Robin George Andrews, Scientific American, 2 June 2021 Some tessera were damaged and others were missing when it was rolled up and transported overseas, but those too were restored. Joseph Serna, latimes.com, 27 May 2018 In the milling street Crivano feels invisible again, a tessera blended into a mosaic. Scarlett Thomas, New York Times, 27 May 2016
Robin George Andrews, Scientific American, 2 June 2021 Some tessera were damaged and others were missing when it was rolled up and transported overseas, but those too were restored. Joseph Serna, latimes.com, 27 May 2018 In the milling street Crivano feels invisible again, a tessera blended into a mosaic. Scarlett Thomas, New York Times, 27 May 2016 Word History
Etymology
Latin, probably ultimately from Greek tessares four; from its having four corners — more at four
First Known Use
1538, in the meaning defined at sense 1