Hellstone

Hellstone is a block that can be found in the Underworld. Mining Hellstone requires a Nightmare Pickaxe or better. It can be smelted into Hellstone Bars at a Hellforge, which requires 3 Hellstone and 1 Obsidian.

Hellstone inflicts the Burning debuff on the player when they touch it, damaging them unless they wear an Obsidian Skull, an Obsidian Shield, or an Obsidian Horseshoe; or drink an Obsidian Skin Potion. It also releases half a block of lava when mined in the Underworld, making it a very dangerous material to mine.

Placing and mining Hellstone will create lava, making it a portable lava source and making lava a renewable resource. However, it only releases lava when mined in the Underworld.

Mining tips
Mining Hellstone can be a significant challenge, not only because Hellstone deals fire damage to the player (which can be nullified by equipping an Obsidian Skull) and will spawn lava in its place when mined; 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 like the Demon Scythe or area effect weapons like the Grenade. The greater threat are the Fire Imps, because they constantly respawn and their fireballs can penetrate walls.

The simplest way to mine Hellstone is to come to Hell with a decent amount of Obsidian Skin Potions and swim around the lava lakes, mining Hellstone without the danger of suffering lava damage, or taking damage from the fire properties of the block itself. Furthermore most enemies won't spawn while you are submerged in lava. As noted above, Hellstone doesn't burn up in lava, so it is perfectly fine if the item isn't picked up immediately by the player. A secondary way to this method is using Lava Waders. The 7 seconds of lava immunity is much longer than you would think if you have a decent pickaxe.

If you do not have Obsidian Skin Potions or Lava Waders, a simple alternative is to specifically mine only the blocks you can immediately pick up, so if there's a block of Hellstone two blocks to the right of you, you can mine it.

Using an artificial Meteor biome
If the Player places 50 pieces of Meteorite Ore or more on the screen where they are mining, they can create an artificial Meteor Biome. In the Meteor biome, all monster spawns are replaced by Meteor Heads, which are more predictable, and tend to do less damage than the naturally spawning creatures of The Underworld. If the Player can withstand the Meteor Head attacks, they can mine Hellstone in relative peace.

If this method is used, the Player only needs to take care to move the Meteorite deposit as they mine away from it. Refer to the Meteorite page for tips on safely mining in the Meteor biome.

Using natural Underworld structures
Another effective method of Hellstone collection is utilizing the naturally spawned structures present in The Underworld. A player may mine Hellstone with little or no danger by mining through Underworld structures' walls in close proximity to Hellstone, being protected from both contact with the Hellstone, and the lava created by the Hellstone.

History
Hellstone Адский камень