Firestone is a held item that helps with evolution. In Pokémon Ruby, Firestone allows the following Pokémon to evolve:
* Eevee into Flareon
* Vulpix into Ninetales
It's important to note that Firestone is only used in the original Ruby, not in Ruby Omega. In Omega Ruby, Eevee evolves into Flareon via a Fire Stone, and Vulpix evolves into Ninetales via a Fire Stone.