What defines the geometric shape of a crystal?

The geometric shape of a crystal is primarily defined by its unit cell arrangement. The unit cell is the smallest repeating unit in a crystal lattice that reflects the overall symmetry and structure of the entire crystal. Each mineral or compound has a distinct unit cell that contributes to its unique crystal habit and symmetry, which in turn influences the external appearance of the crystal.

While the type of bonding and the elemental composition are important in determining various properties of the crystal—such as hardness or cleavage—they do not specifically dictate the geometric shape. Similarly, external factors like temperature can affect the growth and development of crystals, but the inherent geometric shape is rooted in the repeating unit cell configuration of the atoms within the crystal structure. Thus, the arrangement at the unit cell level is the fundamental characteristic that defines the overall geometric form of the crystal.