Moonstone

Moonstone is a very rare ore to find since it can only be found on the surface of the mine and has a low rarity. That seems like it should make it one of the rarest ores in the game but there are other ways to obtain it. Unlocking Unreal Crates and killing Zombos have a chance of a random amount of Moonstone. Moonstone can be used to create the Moonstone Pickaxe which is really useful, especially early on.