Cleavage is the tendency of mineral crystals to fracture into defined crystallographic planes. The direction and the availability of cleavage planes are governed by the structures of crystals. The cleavage is always parallel to the crystallographic planes, and is used by the gem industry to cut the stones. For example, jewel makers will use cleavages in diamonds to cut them into desired shapes. Extra care in designing and cutting of jewels is needed so that the gemstones will not break along the cleavage planes when subjected to external impacts.