Concretions form inside sediments before they harden into rocks, when they are still soft, as minerals within a sediment precipitate, sometimes in successive layers around a nucleus such as a shell or pebble. ... If the planes curve around the concretion, it probably formed early after the deposition of the sediment.

How does concretion occur?

A concretion is a hard, compact mass of matter formed by the precipitation of mineral cement within the spaces between particles, and is found in sedimentary rock or soil. ... They usually form early in the burial history of the sediment, before the rest of the sediment is hardened into rock.

What are concretions made of?

Concretions are masses of mineral matter embedded within rock layers, including limestone, sandstone, and shale. They often take shape when minerals precipitating (settling) out of water collect around a nucleus, such as a pebble, leaf, shell, bone, or fossil.