concretion

In mineralogy, a concretion is a rounded mass of mineral matter that forms within rock, often with a composition different from the surrounding rock. These formations are typically produced by the deposition of minerals from an aqueous solution within the rock.12

Concretions can be composed of various minerals, with silica (chert and flint) being common in limestone and chalk. Other examples include clay ironstone, a mix of clay and siderite, and phosphorites. The process of cementation, where mineral matter precipitates in the pore spaces of sediments, is a key part of how sedimentary rocks form and can lead to the development of concretions. The reverse process, dissolution, can also occur, where cement or grains are broken down.134