The chemical formula for emerald is Be3Al2 (SiO3) 6. An emerald can be described as

An emerald can be described as a compound because it is made up of many different elements.

An emerald can be described as a pure substance because it is a compound. it's a pure substance because it is an element. It's an impure substance because it is chemically combined, an impure substance because it is a mixture.