A minerals crystal form is a set of faces on the mineral that have a definite geometric relationship. Basically, the crystal form we see is the external reflection of the internal structure of the atoms that make up the mineral. A Danish scientist, Steno, was the first to notice that the angle between two adjacent faces of quartz were always the same, no matter where the quartz came from. 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.