Flint forms small rocks often called nodules in chalk rock. The surface of the rock is covered in a white coating but inside the rock is blue-grey or black.
When a piece of flint is struck hard it breaks in a similar way to glass and produces pieces with sharp edges.
People in the Stone Age developed ways of breaking pieces of flint to make tools with sharp edges. Large pieces of flint were used to make axes, medium sized pieces of flint were used to make knives and small pieces of flint were used to make arrowheads.