Magnetite

Magnetite is currentely outranking the Crystal Tier, and was introduced in the 3/10/2018 update, and are very hard to obtain. You must mine a meteor's crash location to get Raw Magnetite, which spawn naturally every 1-2 hours. You can also summon meteors by destroying the moon totem by dropping a spirit key in to the keyhole. Only the Crystal Gaurdian and the Giant Barbarian can drop spirit keys.