From Bulbapedia, the community-driven Pokémon encyclopedia.
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
A small Moss Rock is located in the southwestern section of the Eterna Forest in Sinnoh, to the west of a formation of trees that resembles a sideways L. Eevee will evolve if leveled up anywhere in the forest, except for the Old Chateau and the path that encompasses the outskirts of the forest. As Moss Rock is not present in Kanto and Johto, Eevee cannot evolve into Leafeon at all in HeartGold and SoulSilver. It must be traded from Diamond, Pearl, and Platinum.
A larger Moss Rock is located in the southwestern section of Pinwheel Forest in Unova.
In Kalos, a Moss Rock about the same size of Unova's can be found on Route 20.
In side 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.
In the anime
Unlike its counterpart, which was encountered by May and her Eevee on Route 217, the Moss Rock has yet to be seen in the anime despite Ash and his friends spending a three-episode arc in the Eterna Forest with Cheryl and one episode in Pinwheel Forest with Burgh.