Select the one substance that is not a mineral.

 - Halite is formed in nature. It is a solid.
 - A nutshell is not a pure substance. It is formed in nature.
 - Gypsum is a solid. It is a pure substance.
Compare the properties of each substance to the properties of minerals. Select the substance whose properties do not match those of minerals.
Halite is a mineral.
Gypsum is a mineral.
A nutshell is not a pure substance. But all minerals are pure substances.
So, a nutshell is not a mineral.