Chalk

Chalk is a soft, white, porous, sedimentary carbonate rock, a type of limestone composed of the mineral calcite. When rubbed against most hard surfaces, it will leave markings as the material it is made from breaks down gradually and loosely adheres to the surface. Children find different colors of chalk to be pleasant creative toys, while adventurers find them to be invaluable tools for marking passages in confusing labyrinths and caves.