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Definition:
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A more or less regular periodic array of atoms, molecules, or ions, usually forming a solid. In everyday parlance crystal is used in a bewildering variety of ways, sometimes contradictory. Fine glassware is called crystal, although glass, an amorphous solid, is the antithesis of a crystal. A solid with facets exhibiting external symmetry may be called a crystal, although a solid without such facets may still be a crystal. A pure liquid such as water is said to be crystal clear even though transparency is not an essential property of a crystal.
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