From Bulbapedia, the community-driven Pokémon encyclopedia.
An Ice Rock is an ice-covered rock that enables Eevee to evolve into Glaceon. If an Eevee is leveled up in an area containing an Ice Rock, it will evolve into Glaceon. Even if Eevee's happiness is raised to a maximum, an Ice Rock will still cause it to evolve into Glaceon, rather than Espeon or Umbreon.
A small Ice Rock is located a short way south from Snowpoint City on Route 217 in Sinnoh. It is located in an area where the blizzard is very rough, on the easternmost part of the route. It actually appears that the rock is located in the backyard of a house that is near a female Skier. It is surrounded by long grass.
A larger Ice Rock is located in the basement of Twist Mountain in Unova, just west of Icirrus City. It is located on the lowest floor, past an NPC who gives the player fossils of previous regions.
In the anime
The Ice Rock was shown in a flashback in A Full Course Tag Battle!. May and her Eevee discovered the Ice Rock near Snowpoint City. Unlike the games where an Eevee needs to be leveled up near the Ice Rock or anywhere on Route 217, May's Eevee only needed to touch the Ice Rock and it promptly evolved into a Glaceon.
In side games
Due to the Ice Rock's absence in Pokémon Mystery Dungeon: Explorers of Time and Explorers of Darkness, Eevee evolves into Glaceon through the use of the Frozen Rock. As the Ice Rock is also not present in HeartGold and SoulSilver, Eevee cannot evolve into Glaceon at all unless it is traded to one of the other Generation IV games, Diamond, Pearl, and Platinum.