Crystals are orderly arrangements of pure substances. For example, diamonds are pure carbon, and quartz is pure silicon dioxide.
Crystals form as their component atoms move closer and closer together.
This can occur as pressure and temperature decrease, or as a solvent evaporates. Once the atoms begin to associate, their geometry and chemical activity determine the shape of the crystals that form. In general, the size of the crystals will depend on the amount of substance present in a given space, and how quickly the atoms can associate. The slower the association, the larger the crystal. The fantastic crystal structures in geodes, or hydrolites, form as water evaporates from the internal spaces of the stones.