Select the one substance that is not a rock.

 - An eggshell is made by a living thing. It is formed in nature.
 - Soapstone is not a pure substance. It is a solid.
 - Trachyte is not a pure substance. 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.
An eggshell is made by a living thing. But rocks are not made by living things.
So, an eggshell is not a rock.
Trachyte is a rock.
Soapstone is a rock.