The chemical formula for emerald is be3al2(sio3)6. an emerald can be described as
a pure substance because it is a compound.
a pure substance because it is an element.
an impure substance because it is chemically combined.
an impure substance because it is a mixture.

The correct statement is a pure substance because it is a compound.

Explanation:

A pure substance is defined as the substance which is formed by same or different type of atoms which are chemically combined in a fixed ratio by mass. Compounds and elements are considered as pure substances.

Impure substances are defined as the substances which do not have definite chemical composition. They are basically mixtures of substances which are not chemically combined.

A compound is a substance which is formed by two or more different type of atoms when they chemically combine.

An element is defined as the substance which have only one type of atoms.

Emerald is a compound having chemical formula of . This is considered as a pure substance.

Hence, the correct statement is a pure substance because it is a compound.