The Leaf Stone can only be obtained in Pokemon Emerald by trading a Green Shard with the Diving Treasure Hunter on Route 124 near Mossdeep City. The Leaf Stone is necessary to evolve certain Pokemon.
A shard is a fragment from an ancient tool that can be traded in Generation 3 with the Diving Treasure Hunter. Shards are often found in bodies of water held by Pokemon you encounter. The Diving Treasure Hunter also trades the Water Stone for the Blue Shard, the Fire Stone for the Red Shard, and the Thunderstone for the Yellow Shard.
The Leaf Stone is an evolutionary item with a selling price of 1,050 and a purchasing price of 2,100. The stone was introduced in Generation 1 and is attainable in all versions of Pokemon through different means. Once obtained, the stone must be given to the Pokemon to hold so that it evolves into its more advanced form.
The Pokemon evolved from the Leaf Stone are Grass-type Pokemon. The evolutions requiring the Leaf Stone include Weepingbell into Victreebel, Gloom into Vileplume, Exeggcute into Exeggutor, Nuzleaf into Shiftry and Pansage into Simisage. Other Pokemon evolve with other stones; for example, the Fire Stone is necessary to evolve Vulpix into Ninetales.