A Fire Stone is used to evolve certain Pokémon, such as Eevee. Eevee evolves into Flareon after a Fire Stone is used. Growlithe evolves into Arcanine when a Fire Stone is used. Vulpix evolves into Ninetales with a Fire Stone. If implemented, Pansear will evolve into Simisear with a Fire Stone.
Requires 9 Fire Stone Shards, found anywhere under lava or the roofs of lava caverns, including the Nether. (Although, I've never seen a firestone in the Nether... Maybe in the next update.)