Argillite is a fine-grained metasedimentary rock composed of clay particles.  It forms when a sedimentary clay or mud is cooked by exposure to intense heat and pressure within the Earth’s crust.  Argillite is intermediate between shale and slate, but it does not possess cleavage planes, which makes the dense and hard varieties suitable for stone tool-making.  Argillite and mudstone are similar in origin, texture, and knapping quality, but mudstones are not metamorphosed.  Argillite can occur in a variety of colours.