Crystal

Quartz crystal

A crystal is a solid whose molecules (or atoms) are arranged in a repeating pattern. Sometimes pressure needs to be applied to a substance for crystals of it to form.

Different substances form different types of crystals. The study of the various kinds is crystallography. Well-known substances that form crystals are table salt (whose crystals are cubes) and quartz.

Atoms (as ions) in a crystal of sodium chloride. The blue ions are sodium and the green ions are chloride.