The type of chalk used in gyms is called magnesium carbonate (MgCO3), a white solid that occurs in nature as a mineral. It is available in three forms: as a powder, a solid block that looks similar to soap, and a liquid. The powder form is the messiest, so using block chalk and liquid chalk is preferable if you train in a gym that forbids messy powder chalk. Liquid chalk transforms into solid chalk seconds after it is applied and is legal for use in all competitions.