Ash Block

It is the floor and ceiling of the underworld, so you end up with thousands of it just from carving out a hellstone run. That abundance is the whole argument for using it. It is also the only ground Fireblossom will grow on, which is the practical reason to haul a stack back home.

Corruption, Crimson and Hallow cannot convert it. In hardmode that makes it a cheap wall material for sealing off a biome or lining a hellevator, and unlike obsidian or dungeon brick you already own more of it than you can carry.

In For the Worthy worlds it behaves like sand. Break one block and neighbors can come down on top of you, in the same way cracked dungeon brick does. Do not stand under a ceiling of it there while you dig.