Chalk is a soft form of the mineral calcium carbonate. Chalk is relatively resistant to erosion and slumping and so forms tall steep cliffs where chalk ridges meet the sea. Chalk hills, known as chalk downland, usually form where bands of chalk reach the surface at an angle.

For more on chalk landscapes see: Downland.

The substance known as "blackboard chalk", often supplied in sticks about 2 inches (or 5 cm) long and used for drawing, especially on blackboards and sidewalks, is not actually chalk: it is made from gypsum, calcium sulfate. Similarly, the "chalk" used by tailors is usually made from talc, magnesium silicate.

See also: List of minerals