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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 pressure needs to be applied to a substance for crystals of it to form, for example diamond. Crystals are found naturally or can be made artifically.
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.