Many minerals split of break readily along certain well-defined planes. This is referred to as 'Cleavage'.

Where a mineral breaks in an irregular manner, it is called 'fracture'. It is the type of surface left after the break and seldom has any relationship to crystal structure.

Cleavage occurs due to weaknesses in the binding force of parallel planes in the atomic structure of a mineral. The planes along which separation takes place are called 'cleavage planes' and the resultant surfaces are called 'cleavage faces'. Since the cleavage planes are determined by a mineral's atomic arrangements, which are also responsible for it's external form, cleavage will be parallel to it's external crystal faces.

Cleavage descriptions are defined by the particular crystal face to which it is parallel, for example, cubic cleavage implies cleavage along planes parallel to a cube. As a cubic crystal has three pairs of faces, it will also have three sets of cleavage planes. Similarly, an octahedron will have four cleavage planes, and so on.

Cleavage is described according to it's perfection as either perfect, good, fair, or poor. Galena, for example, has three perfect cleavage planes, meaning it will cleave easily and exactly in three directions. Another mineral might have perfect cleavage in one direction, fair cleavage in another, and poor cleavage in another. Some minerals have no cleavage at all. Some well defined specimens may have much more cleavage than a fine grained or massive form of the same material. Jade is one of the bvest example of a mineral with very poor, imperfect cleavage. It is for this reason that is is so perfect for very fine carving and has a history of use in China for purposes that other cultures were using metals for.

When a mineral breaks in a direction other than that of a cleavage plane, it is known as a fracture. Minerals with perfect cleavage will tend to part along cleavage planes making it difficult to determine their fracture. If you hit a piece of calcite with a hammer it will tend to break into smaller and smaller rhombohedrons rather than show any fracture. 

Types of fracture are;

• Conchoidal - the surface is covered by coincentric waves, resembling the inner-wall of a shell. Examples are chips on glass or quartz.
• Subconchoidal - same as conchoidal but less distinct.
• Even - the break is quite smooth and flat.
• Uneven - the surface is covered by small elevations and depressions.
• Hackly - the surface is pointed and rough. Examples are gold and silver.
• Splintery - breaks into splinters and fibres. Examples are jadeite and nephrite.
• Earthy - breaks into pieces as dirt or clay.

A stone is 'brittle' if it fractures easily and 'tough' if it is difficult to fracture.

A number of minerals will break across planes usually caused by deformation or twinning. This is known as 'parting'. Although it might resemble cleavage, it only actually cleaves across a limited number of widely spaced planes with irregular fractures between them..