Concretion

A concretion is a solid mineral inclusion within a rock strata that is oval or spherical in shape. These objects form within layers of sedimentary strata that have already been deposited, where the grains are cemented together by secondary cementation, i.e.: the cementation occurs due to processes independent from the primary cementation in which the layers of sedimentary rock were adhesed together.

Concretions may outwardly resemble fossils or rocks that look as if they do not belong to the strata in which they were found. Occasionally, concretions contain a fossil either as its nucleus or as a component that was incorporated during its growth, but concretions are not fossils themselves.

The reasons for growth of concretions are poorly characterized.