Crystals are solids that form by a regular repeated pattern of molecules connecting together. In crystals, a collection of atoms, called the Unit Cell, is repeated in exactly the same arrangement over and over throughout the entire material.
- An orderly and symmetrical atomic structure and a definite chemical composition.
- A definite external geometrical shape bounded by plane faces.
- Physical (and optical) properties which vary with direction (except for the isometric minerals).
Only if a mineral is allowed to grow without interference from other minerals will it form crystals. Minerals "grow," or crystallize, from many types of solutions. They may precipitate from evaporating sea water or crystallize from magmas when lava cools. While growing, minerals may develop a distinct crystal form with smooth, flat planes called crystal faces. The geometric pattern of the crystal faces reflects the internal atomic arrangement of the crystal structure. This is one of the most important things about a mineral.