Chalk /ˈtʃɔːk/ is a soft, white, porous sedimentary carbonate rock, a form of limestone composed of the mineral calcite.Calcite is calcium carbonate or CaCO 3.lint (a type of chert unique to chalk) is very common as bands parallel to the bedding or as nodules embedded in chalk. It is probably derived from sponge spicules or other siliceous organisms as water is expelled upwards during compaction.