A crystal is a solid whose molecules (or atoms) are arranged in a repeating pattern.

Crystals are made from fluids by phase change. Usually the fluid is a liquid but snow is crystals precipitated from air. Crystallization can be by freezing from a simple fluid or from a solution. Sometimes heat and pressure needs to be applied to a substance for crystals to form. Diamond is a carbon crystal formed under heat and pressure. Crystals are found naturally or can be made artificially.

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.

File:Sodium chloride crystal.png
Atoms (as ions) in a crystal of sodium chloride (table salt). The blue ions are sodium and the green ions are chloride

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