(noun) A hard earthen substance that can form large rocks.
"Dreams are the touchstones of our characters."
"The stone that the builder refused shall be the head corner stone."
"Never give up. In the end, even the most harmless drop ruins the hardest stone."
"I alone cannot change the world, but I can cast a stone across the waters to create many ripples."
"Our wretched species is so made that those who walk on the well-trodden path always throw stones at those who are showing a new road."