So I just caught an Eevee. Now, I need 25 candies to evolve it. The fact is that in the GameBoy color versions, that Pokémon needed a stone to evolve instead of experience. Depending of the stone he was fed with (thunder, water or fire), he was evolving in a different Pokémon.
Is the stone feature also in Pokémon Go? If not, what decide the evolution of this Pokémon?
Bonus point : If it’s possible, how can I get a stone?
Evolution stones as you know them (water, fire, etc.) don’t exist in the current version of Pokémon GO. By default, when you evolve an Eevee in Pokémon GO, the evolution is chosen randomly from the first set of potential “eeveelutions” (Vaporeon, Jolteon, Flareon).
To get other Eeveelutions like Espeon/Umbreon, other requirements must be met. See @MBorg’s answer for more details.
There is also a method for forcing a specific evolution by giving your Eevee a certain nickname before it evolves. See @Brant’s answer for more details.