Cleavage is the tendency of minerals to break along definite crystallographic structural planes. Cleavage planes are like the grain boundaries in metals, but they are much more regular. Cleavage planes are caused by the way the atoms in the mineral are arranged.
The quality of a mineral's cleavage refers to both the ease with which the mineral cleaves and to the character of the exposed cleavage surface. The quality of a sample's cleavage is typically described by terms such as 'eminent,' 'perfect,' 'distinct,' 'difficult,' 'imperfect,' or 'indistinct.'
'Eminent' cleavage describes the case in which cleavage always occurs readily and is in fact difficult to prevent from occurring. The mineral mica, for example, cleaves readily into thin, flat sheets.
A mineral which demonstrates 'perfect' cleavage breaks easily, exposing continuous, flat surfaces which reflect light. Fluorite, calcite, and barite are minerals whose cleavage is perfect. 'Distinct' cleavage implies that cleavage surfaces are present although they may be marred by fractures or imperfections. 'Difficult' or 'indistinct' cleavage produces surfaces which are neither smooth nor regular; samples possessing such cleavage tend to fracture rather than split.
Cleavage may be determined by the examination of surfaces which have actually broken. It may also be determined by inspection of the interlacing systems of cracks which permeate the structure of certain specimens. These systems of cracks are beautifully apparent within transparent crystals such as fluorite or calcite.