Select the one substance that is not a rock.

 - Granite is not a pure substance. It is not made by living things.
 - Syenite is a solid. It is formed in nature.
 - A marble is made by humans. 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.
Syenite is a rock.
A marble is made by humans. But rocks are not made by living things.
So, a marble is not a rock.
Granite is a rock.