Question

what characteristic makes a substance a mineral?
it has a definite chemical composition.
it is lustrous.
it is only an organic compound.
it is found in certain rock strata.

Explanation:

A mineral has a definite chemical composition. Luster is a physical property but not the defining characteristic. Minerals are inorganic, not organic compounds. While they can be in rock strata, the key is the definite chemical composition.

Answer:

It has a definite chemical composition.