Cleavage in minerals refers to the way some minerals break along certain lines of weakness in their structure). When part of a crystal breaks due to stress and the broken piece retains a smooth plane or crystal shape, the mineral has cleavage. A mineral that never produces any crystallized fragments when broken off has no cleavage. Cleavages are described in terms of their quality - how smoothly the mineral breaks - and their difficulty - how easy, or how hard, it is to produce the cleavage. The quality of cleavages are perfect, imperfect, distinct, good, fair, and poor. The difficulty is described as easy, hard, and difficult to produce.
The main thing that needs to be considered in the identification of minerals is whether or not a sample has a cleavage – many minerals don’t, breaking without producing smooth flat surfaces. Next is whether or not there are two or more cleavage surfaces present at angles to one another and, if so, the quality of the various cleavages. Where two or more cleavage surfaces are present, it then becomes important to figure out which crystal form they represent – cubic, prismatic, and so on.
Parting is a characteristic that is similar to cleavage and occurs in minerals that are crystallographically twinned or which have been stressed by pressure. It is usually not as well, or regularly, developed as cleavage surfaces - resembling an irregular fracture.
