Ancient Chisel

Mining speed only earns an accessory slot while you are actually digging. Swap this in for a hellevator, a long ore vein or an arena excavation, and take it back off the moment you go fight something, because it does nothing at all in combat.

It stacks with a Mining Potion, and the two together make a real difference over an hour of tunneling. It will not combine with the Hand Of Creation, which is the accessory it eventually becomes part of, so once that is built there is no reason to keep this one equipped alongside it.

Sandstone Chests in the upper reaches of an Underground Desert are the direct route. Fishing an oasis is the patient route, since the desert crates can hand one over too. If you are already fishing that biome for something else, expect to see one eventually without going out of your way.