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 obtained from Leaf Stone ore, found within the stone directly under trees. Each Leaf Stone ore will drop one shard.
Leaf Stones are also used to make Rose Incenses, Grass Gems, Leaf Stone tools, and Leaf Stone armor.