Opal

Opal is a lot like the ore Moonstone since it only spawns at the top levels but is a lot easier to get. You can still just mine forward at the top and find it if you want to get it that way. Opal also drops from Zombies and can be found in Regular Crates. Opal is a very useful ore since it is used to make the Teleport Pad, which is very good, and the Opal Pickaxe, Olivine Pickaxe, and Azure Pickaxe.