A concretion's a volume of sedimentary rock in which a mineral cement fills the porosity (for example the spaces between the sediment grains). Concretions are often ovoid or spherical in shape, although irregular shapes also occur. The word 'concretion''s derived from the Latin con meaning 'together' and crescere meaning 'to grow'. Concretions form within layers of sedimentary strata that've already been deposited. They usually form early in the burial history of the sediment, before the rest o… (More on Concretion)