Select the one substance that is not a rock.

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