Headcanon #64
Evolutionary stones are used to evolve Eevee when it's not in the right environment for the Trainer's desired evolution. When evolved naturally, all Eevee evolve with friendship. This makes Eevee a rather key component to understanding Pokémon evolution since its evolution methods involve environment, friendship, and stones.
An Eevee with a high friendship, raised in an area that is frequented by thunderstorms will evolve into a Jolteon. However, the same Eevee can evolve into a different evolution with the use of a stone, regardless of friendship bonds.
Curiously, there are three branches of Eevee evolutions that cannot evolve with stones: Umbreon, Espeon, and Sylveon. These three evolutions in particular continue to mystify researchers as they try to understand the circumstances that causes Eevee to evolve into each of these, and how it defers from the environment standards of other Eevee evolutions. These are the only evolutions you might find mixed in with other evolutions in their environments.
