Cleavage is the tendency of some crystals to split along definite planes. Crystals with cleavage tend to break parallel to certain atomic planes, creating relatively smooth surfaces. The presence of cleavage indicates a weaker bonding across the planes of a crystal or a greater spacing between the planes. Some crystals are without cleavage.
The terms which describe cleavage are quality, ease of production and crystallographic form. Quality is expressed as perfect, distinct, good, imperfect and poor. Ease of production refers to how easily a crystal splits along its cleavage planes and is expressed as easy, difficult, etc. The crystallographic form describes the crystal form to which the cleavage plans are parallel. Cleavage in Calcite is perfect, easy, rhombohedral; indicating that perfect is the quality, easy is the ease of production and rhombohedral is the crystallographic form. The cleavage of a diamond is perfect, octahedral. Cleavage in Beryl is poor, pinacoidal.
Cleavage is of particular importance in the cutting of gemstones. Gems with perfect cleavage must be faceted, set and worn carefully because a sharp impact to the gem along a cleavage direction may easily split the gem. Gem cutters must be very aware of cleavages because it is virtually impossible to polish a gemstone surface that is parallel to a cleavage plane.
Cleavage, along with fracture, can be of important diagnostic value in determining a type of unknown mineral.
Fracture is the way a mineral breaks other than along cleavage planes. Please see the Fracture page for more information.