Pearlstone Block

Hallow spreads out of this block into nearby stone, sand, ice and grassy dirt, so a vein you ignore in your cavern will keep chewing outward. If there is a jungle or a mushroom patch you care about nearby, cut a gap of a few tiles between them or line the border with something inert. Green Solution through the Clentaminator reverses it in bulk, and Vile Powder handles small patches by hand.

Below the cavern line it sprouts Crystal Shards on any exposed face, and they come back after you harvest them. A single corridor cut through hallowed stone is a shard farm that never runs dry, which pays for mana potions and crystal bullets for the rest of the run. Cut it wide and light it, then walk the loop whenever you pass.