The chemical formula for an emerald is Be sub 3 Al sub 2 open paren SiO sub 3 close paren sub 6 An emerald can be described as

The formula Be₃Al₂(SiO₃)₆ shows that an emerald consists of different elements chemically combined in a fixed ratio. A substance with a fixed composition is a compound, and compounds are pure substances. Therefore, an emerald can be described as a pure substance because it is a compound.

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