Dungeon Bricks

Dungeon Bricks are a non-craftable type of Brick that Dungeons are constructed out.

Description
On world generation one of a variety of colors of brick is chosen to create the entirety of the dungeon out of. They are twice as strong as normal bricks and are immune to explosions. A player can get them by mining, but they become relatively harder to mine in lower layers of the dungeon because of the monsters.

An uncraftable world generated item, there is a huge (but finite) number of bricks generated per world.

As of patch 1.0.4 a Nightmare Pick is required to mine it.