oolith

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.

A spherical granule of which oolite is composed, formed by concentric accretion of thin layers of a mineral around a core. Calcium carbonate (limestone) is the most common mineral that forms ooliths, but they may also form from other minerals such as dolomite and silica.

Oolite.