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Pinwheel Forest's Moss Rock
Eterna Forest's Moss Rock in Platinum
A Moss Rock is a moss-covered rock that enables Eevee to evolve into Leafeon. If an Eevee is leveled up in an area containing a Moss Rock, it will evolve into Leafeon. Even if Eevee has reached other evolutionary conditions, the Moss Rock will still cause it to evolve into Leafeon rather than Espeon, Umbreon, or Sylveon.
In the games
Eevee will evolve into Leafeon if leveled anywhere in these locations, as long as the player is in the same area as the Moss Rock. Eevee will not evolve into Leafeon if leveled up in the Old Chateau or the path that encompasses the outskirts of Eterna Forest. As the Moss Rock is not present in Kanto or Johto, Eevee cannot evolve into Leafeon at all in HeartGold and SoulSilver; it can only be obtained by trading it from Diamond, Pearl, and Platinum.
In spin-off games
Due to the Moss Rock's absence in Pokémon Mystery Dungeon: Explorers of Time, Darkness and Sky, Eevee evolves into Leafeon through the use of the Mossy Rock. It evolves by the same method in Gates to Infinity.