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

I think the best answer is the first option. An emerald can be described as a pure substance because it is a compound. A pure substance is defined as a matter having definite and constant composition all throughout a sample. For this case, emerald is chemically held together so at whatever point you get a sample, it would still be emerald.

An emerald can be described as a pure substance

because it is a compound.

increased stability in the completion of fluorine's outer shell.