Earth key - unlocks bronze chests. without this key, don't bother to try to unlock them unless you're prepared to face a mimic. found on juraika, in the burkaqua chief's house.
Star key - unlocks silver chests, which often contain better items than bronze chests. silver chests never have mimics. found on vedan, in angela's bar.(requires earth key)
Sun key - unlocks gold chests, which usually contain items of the highest quality/rareness. gold chests never have mimics. found on rosa, in the old man's house in johannasberg, after a cutscene. (requires star key)