Science & Nature - Concretions: how mineral cement forms spherical rock oddities

A concretion is a compact, hardened geological mass that forms when mineral cement precipitates into the spaces between particles in sedimentary rock or soil. Often developing early in sediment burial before surrounding layers turn to stone, concretions tend to be tougher and more resistant to weathering than their host rock. Because of their frequently spherical or ovoid shapes and unusual compositions, historical observers frequently mistook them for dinosaur eggs, extraterrestrial debris, or pseudofossils, making them enduring objects of geological curiosity since the eighteenth century.

Source: Wikipedia