Crimstone Block

It needs a decent pickaxe before hardmode, which is why a fresh character standing at the lip of a chasm cannot simply tunnel sideways into it. Either upgrade first or bring explosives, which ignore the requirement entirely and are usually the faster answer when all you want is to reach the hearts at the bottom.

Purification Powder flips it back to plain stone a patch at a time. That is fine for clearing a pocket near your base and useless against a hardmode outbreak, since the block converts nearby stone on its own once the world evil starts spreading. Containment is a dug gap, not powder.