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. Cubic cleavages form cubes (example, halite). Rhombohedral cleavages form six-sided prisms (example, calcite). Basal cleavages occur along a single plane parallel to the base of the mineral (example, topaz).
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. Some minerals cleave perfectly in one direction and poorly in others. For example, gypsum cleaves perfectly on one plane and poorly along two others.