The Moss Rock and Ice Rock are ice/moss covered rocks that enable an Eevee to evolve into Glaceon or 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.
If an Eevee is leveled up in an area containing an Ice Rock, it will evolve into Glaceon. Even if Eevee has reached other evolutionary conditions, the Ice Rock will still cause it to evolve into Glaceon rather than Espeon, Umbreon, or Sylveon.