A sledgehammer is a tool that is fashioned on a Carpentry Bench that has been fitted with a carpentry blade. It takes 1 board and 5 iron to assemble a sledgehammer.
A sledgehammer can be used to smash medium stones into gravel. While holding the sledgehammer, click on a medium stone on the ground to give it a good hearty whack. Successive hits will break the stone into smaller and smaller pieces (cracked stone, broken stone, split stone, smashed stone) and eventually into gravel.