Concretion

A hard, rounded mass that formed by the precipitation of mineral cement within the spaces between sedimentary particles and enclosing a nucleus, such as a shell or fossil. They range in diameter from centimeters to meters. Concretions are found in sedimentary rock or soil and often ovoid or spherical in shape, although irregular shapes do also occur. The word “concretion” is derived from the Latin “con” meaning “together” and “crescere” meaning “to grow”.