Ebonstone Block

Nothing below a Nightmare Pickaxe will break it, which is awkward, because the first place you meet it is the chasm walls in the Corruption, long before you own one. Explosives will do it. So will Purification Powder: thrown at the wall, it converts the block back into ordinary Stone, and stone comes apart with the copper pickaxe you started the world holding. Carry powder on the first chasm dive and the pickaxe gate stops applying at all.

It is the Corruption's take on stone, the same way Crimstone belongs to the Crimson and Pearlstone to the Hallow. Anything you already know about handling one of those transfers straight across.

This is the block the Corruption actually travels on. Stone in its path converts, and each newly converted block then converts more around it, so the biome creeps sideways through the cavern layer along whatever unbroken run of stone you left in place. Containing it is a job of cutting every connected line of stone underground, not fencing off the purple you can see on the surface. Pushing the border back is the same work in reverse: Green Solution out of a Clentaminator turns it to plain stone far faster than you could ever mine it.

Deathweed roots in it, so a deliberate patch in a planter box back at base is a herb farm rather than an infection you forgot about. Past that it is the base material for the whole corrupt building set: the dark torches, the purple wall varieties, Ebonstone Brick, Lesion Block. Several of those want a bench standing in Ecto Mist, which means a graveyard, so decide where you are building before you carry a stack across the world.

Ebon is an old word for black. The name means black stone and nothing more.