The chemical formula for emerald is be3al2(sio3)6. an emerald can be described as

An emerald can be described as a pure substance because it is a compound. Compounds are considered pure substances because they cannot be broken apart by any mechanical or physical process. A compound is any molecule that is made from two or more atoms of the same or different elements. Water is also considered a pure substance. it is made up of one oxygen atom and two hydrogen atoms. A water molecule cannot be separated by any mechanical or physical process.