Different crystals have different shapes and forms because of the:
- stacking of the unit cells
- internal arrangement of the atoms
- the environment in which they grow
Crystal shapes (morphology) can be a valuable tool in mineral identification.
The study of crystals and the laws that govern their growth, external shape, and internal structure is called crystallography.
A crystal may possess only certain combinations of symmetry elements. Only 32 possibilities exist, and these are the 32 crystal classes that are grouped into six crystal systems. Every mineral belongs to one of these crystal classes.
To describe crystals, imaginary lines are used intersecting at 0 (the origin). These are specific to the various crystal systems, intersecting at given angles and being of given lengths specific to each crystal system.
To discuss the six crystal systems, we have to describe what are called crystallographic axes. There are three axes: a, b, and c. The axes pass through the center of the crystal, and, by using them, we can describe the intersection of any given face with these three axes.