Steno looked at other minerals and discovered that they too had sets of angles between faces that never varied. This observation became known as Steno's Law . Many minerals have a tendency to break or split along preferred lines, known as cleavage . The reason the minerals break along these cleavage lines is that the bonds between the atoms that make up the mineral are weakest there. Cleavage is identical for a given mineral from sample to sample, making cleavage a good diagnostic tool.
Minerals can have one cleavage direction, meaning they only break along one plane.
Muscovite is an excellent example of a mineral with a single cleavage plane. Minerals can also have two, three, four or six cleavage planes. It takes some practice to identify cleavage planes in minerals. It is not immediately apparent, but is one of the most diagnostic tools for mineral identification. Fracture is how a mineral breaks when not controlled by cleavage.