Select the one substance that is not a rock.

 - Slate is not a pure substance. It is a solid.
 - Syenite is formed in nature. It is not a pure substance.
 - Wool is made by living things. It is formed in nature.
Compare the properties of each substance to the properties of rocks. Select the substance whose properties do not match those of rocks.
Slate is a rock.
Syenite is a rock.
Wool is made by living things. But rocks are not made by living things.
So, wool is not a rock.