Evolves certain kinds of Pokémon.
Use this on Eevee for Glaceon or Alolan Sandshrew for Alolan Sandslash. Nice that Glaceon doesn't require some specific icy rock location in this game.
Look for these in cold areas. Glaceon hits hard but don't expect it to take hits well - its defensive stats are pretty bad. You'll want something to switch into if things go wrong.
Game:Polished Crystal