Concretionary stone

Rock characterized by, consisting of, or producing concretions; concretions are hard, compact aggregates of mineral matter, subspherical or irregular in shape, formed by precipitation from water solution around a nucleus, such as a shell or bone, in a sedimentary or pyroclastic rock. Concretions are generally different from the rock in which they occur, and represent a concentration of some minor constituent of that rock. Concretionary stones include flint, chert, and nodules of impure limestone which sometimes occur in clay.