A Leaf Stone is one of the evolution stones. It can be used to evolve certain Grass-type Pokémon:
It can be used by sending out a compatible Pokémon and right-clicking the Pokémon with the Leaf Stone. The Pokémon will evolve into the corresponding evolution.
A Leaf Stone does not evolve Eevee into Leafeon. A Moss Rock is required to do this.
Crafting
A Leaf Stone is crafted with nine Leaf Stone shards. Leaf Stone shards can be found within the leaves of trees. They resemble leaf blocks with bright yellow patches on them. Each Leaf Stone ore will drop one shard, so it is necessary to find nine ore.
Leaf Stones are also used to make Rose Incenses and Grass Gems.