Hellstone

Hellstone and its products are the highest tier items currently available in the game and is only found in the deepest layer of the world, The Underworld. Mining Hellstone requires a Nightmare Pickaxe or better. It can be smelted into Hellstone Bars at a Hellforge, which requires 6 Hellstone and 2 Obsidian. As most other ores and soils, it can also be crafted into a Hellstone Brick at any furnace.

Hellstone deals fire damage to the player when they touch it, unless they wear an Obsidian Skull.

Hellstone is invulnerable to all forms of explosives including Dynamite blasts.

Mining Tips
Mining Hellstone can be a significant problem, not only because Hellstone deals fire damage to the player (which can be nullified by an Obsidian Skull); but mainly because of the dangerous monsters of The Underworld. While Bone Serpents don't appear very often, they can be a problem and should be fought like one would fight any worm monster, preferably using very long range weapons such as the Vilethorn. The greater threat are the Fire Imps, because they constantly respawn and their fireballs can penetrate walls.

One approach to mining Hellstone can be to just drop down to the Hellstone / Ash "floor" and start digging out the Hellstone. Since all Pickaxes are pretty fast and swing continuously, this should deflect most fireballs and kill most Fire Imps that stand in front of the player. One should take care that the Fire Imps spawn above them, by mining as narrowly as possible and preferably straight downwards. With decent armor, an Obsidian Skull and one or more Band of Regeneration, a player should be able to mine a decent amount of Hellstone before they are killed by the Fire Imps or a Bone Serpent. Also, using a fast pick axe will help deflect fireballs. One must keep in mind, however, that Hellstone Bars are very expensive, and that 6 pieces of Hellstone and 2 pieces of Obsidian are required to forge only 1 bar.

Once you are at the very bottom of the world, a way to effectively suppress Fire Imps spawns - and get Hellstone with less effort and danger - is to deny Fire Imps the right to spawn by not letting them have enough space to do so. As mining Hellstone gives also a lot of Ash Blocks, one can use these blocks to fill the space left by mining. Note that you don't need to fill the space so much, as Fire Imps seem to require a target space of 4 blocks high by 3 blocks wide to teleport into a location. Having 3 blocks high tunnels is enough to prevent spawning.