Claystone ( /ˈkleɪstoʊn/) is a geological term used to describe a clastic sedimentary rock that is composed primarily of clay-sized particles (less than 1/256 millimeter in diameter).
Claystone does not refer to those rocks that are laminated or easily split into thin layers called clay shales.
Claystones are distinct from mudstones, which are partly hardened muds that slake when wetted; claystone is fully hardened material.