Select the one substance that is not a rock.

 - Chalk is a solid. It is formed in nature.
 - A nutshell is made by a living thing. It is a solid.
 - Quartzite is a solid. It is not a pure substance.
Compare the properties of each substance to the properties of rocks. Select the substance whose properties do not match those of rocks.
Quartzite is a rock.
A nutshell is made by a living thing. But rocks are not made by living things.
So, a nutshell is not a rock.
Chalk is a rock.