The Collaborative International Dictionary of English v.0.48:

Abaculus \A*bac"u*lus\ ([.a]b*[a^]k"[-u]*l[u^]s), n.; pl.
   Abaculi (-l[imac]). [L., dim. of abacus.] (Arch.)
   A small tile of glass, marble, or other substance, of various
   colors, used in making ornamental patterns in mosaic
   pavements. --Fairholt.
   [1913 Webster]