From Bulbapedia, the community-driven Pokémon encyclopedia.
A Moss Rock is a moss-covered rock that enables Eevee to evolve into Leafeon. If an Eevee is leveled up in an area contaning a Moss Rock, it will evolve into Leafeon; even if Eevee's happiness is raised to a maximum, the Moss Rock will still cause it to evolve into Leafeon rather than Espeon or Umbreon.
A small Moss Rock is located in the south-western section of Eterna Forest in Sinnoh, to the left 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.
A larger Moss Rock is located in the southwestern section of Yaguruma Forest in Isshu.
Unlike its counterpart on Route 217, which was encountered by May and her Eevee, Eterna Forest's Moss Rock has not been seen in the anime despite Ash and his friends spending a three-episode arc in Eterna Forest with Cheryl.
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. As the Moss Rock is also not present in HeartGold and SoulSilver, Eevee cannot evolve into Leafeon at all unless it is traded to one of the other Generation IV games, Diamond, Pearl, and Platinum.