The hardness scale was established by the German mineralogist, Friedrich Mohs. The Mohs hardness scale places ten common or well-known minerals on a scale from one to ten. One is the softest mineral and ten is the hardest. When a mineral sample is broken with a hammer, it breaks along planes of weakness that are part of its crystalline structure. These breaks are cleavages.
Some minerals break only in one direction.
Others break in two or more directions. Some common forms of cleavage are cubic, rhombohedral, and basal. If a mineral breaks easily and cleanly in one or more directions, its cleavage is considered perfect. For example, calcite cleaves perfectly along three planes. As the quality of the break decreases, cleavage may be described as good, distinct, and poor or none.