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 at least a Nightmare Pickaxe which is crafted using Demonite Bars dropped by bosses. 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 forge.

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

Mining Tips
Mining Hellstone can be a significant problem, not only because Hellstone deals fire damage to the player (which can be mitigated with 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 continously, this should deflect most fireballs and kill most Imps that stand in front of the player. One should take care that the Imps spawn above one, 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 he is killed by the Imps or a Bone Serpent. One must keep in mind, however, that Hellstone Bars are very expensive, and that 600 Hellstone are required to forge only 100 bars.