o·o·lith 
(ōə-līt′)
(ōə-līt′)n.
A small, round grain consisting of concentric layers of calcium carbonate, silica, or dolomite that have precipitated around a shell fragment, sand grain, pellet of alga, or other central object. Also called ooid.
[OO- (since ooliths resemble fish roe in texture and appearance) + -LITH.]
o′o·lithic adj.