Billiard chalk is a powdered substance placed on the tip of a cue stick to increase the friction between the tip of the cue and the cue ball. Cue "chalk" is not actually chalk (calcium carbonate) at all, but a compound of silica and aluminum oxide. Usually it is a cube of compressed blue powder, wrapped in a paper on all but one side, and is applied in a manner similar to lipstick on the mouth.