Select the one substance that is not a rock.

 - Wool is made by living things. It is not a pure substance.
 - Scoria is formed in nature. It is a solid.
 - Obsidian is a solid. 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.
Obsidian is a rock.
Scoria is a rock.
Wool is made by living things. But rocks are not made by living things.
So, wool is not a rock.