Select the one substance that is not a mineral.

 - Sphalerite is a pure substance. It is not made by living things.
 - Steel is made in a factory. It is not a pure substance.
 - Pyrite is formed in nature. It is not made by living things.
Compare the properties of each substance to the properties of minerals. Select the substance whose properties do not match those of minerals.
Steel is made in a factory. But all minerals are formed in nature.
Steel is not a pure substance. But all minerals are pure substances.
So, steel is not a mineral.
Pyrite is a mineral.
Sphalerite is a mineral.