What is a concretion in geology ?

Concretion : definition

A concretion is a thickening by accretion of material around a nucleus or surface. The concretions are usually formed by successive deposits of chemical origin (precipitation of dissolved salts) or biochemical (intervention of living organisms which fix mineral substances).
The concretions are known in very many geological environments (caves, veins, stratiform deposits, karsts, etc...). The minerals deposit there in concentric layers giving a concretionated texture.