Rocks have been shown in several significant roles.
Truman Oatley used to own a lucky rock that, for some reason, made him feel braver. This rock was lost in Martha and Truman Get Lost, and then he thought he'd found it and became brave, but then Martha found his real lucky rock, which had his name written on it, and he decided he didn't need it to be brave, so he threw both rocks in the water.
In The Trouble with Teddy, Teddy ate a (round) rock because he was hungry and wondered if it would taste of chicken, and he was a bit unintelligent so he didn't stop to think of the consequences. It made him ill so he had to go to the vet, but he was fine after an operation.