Solid body bounded by natural plane faces that is the external expression of a regular internal arrangement of constituent atoms, molecules or ions.

The particles in a crystal occupy positions with definite geometrical relationships to each other, forming a kind of scaffolding called a crystalline lattice. On the basis of its chemistry and the arrangement of its atoms, a crystal falls into one of 32 classes; these in turn are grouped into seven systems according to the relationships of their axes.

Differences in the physical properties of crystals sometimes determine the use to which they can be put in industry.