A Leaf Stone is one of the Evolution Stones. It can evolve Gloom into Vileplume, Weepinbell into Victreebel, and Exeggcute into Exeggutor. It can be used by sending out the compatible Pokémon and right-clicking the Pokémon with the Leaf Stone. It will evolve into the corresponding evolution.
A Leaf Stone is crafted with 9 Leaf Stone Shards. Leaf stone shards can be found on trees in forests and jungles. They are leaf blocks with bright yellow patches on them. Each Leaf Stone Ore will drop 1 shard, so it is necessary to find nine ore.