Hellstone

Hellstone is a block that can be found in the Underworld. Mining Hellstone requires a Nightmare Pickaxe, its Crimson world equivalent the Deathbringer Pickaxe, or better. It can be smelted into Hellstone Bars at a Hellforge, which requires 3 Hellstone (4 on mobile) 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, an Obsidian Horseshoe, or an Ankh Shield; 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, or any accessory made with it) 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 the Underworld 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 the Spawning Limits
There is a hidden limit to the number of spawned enemies that can be spawned at once. You can use this to your advantage by simply not killing the enemies that are not a threat to you, but keeping them close enough to you that they don't despawn. This functionally prevents any actually threatening enemy from spawning, letting you dig at your own leisure.

The simplest way to accomplish this is to place dirt (or any other block you have in huge quantities) tunnels or dig tunnels in the ash while letting demons and bats fly around you just above. Don't kill them.

If a bone serpent spawns, it will be trouble, but you have time to regenerate once it is dead. If an imp spawns, either destroy its projectiles until it decides to teleport inside your tunnel, and kill it there, or use the Vilethorn to kill it from inside your tunnel.

Keep in mind that creatures need a certain amount of space to spawn - imps take a 2x3 tile area to spawn, and bats take 2x2. If you are worried about spawns in your tunnels, either place dirt/stone walls to prevent that, or else place dirt behind you so that no space larger than 2x2 is remaining.

The trick is to not go so far away that the demons or bats can despawn - it may actually be worthwhile to dig them a separate little access tunnel of their own, as well.

Once your spawn limits are hit, you can dig in peace using the "reservoir" method to find places to put the excess lava.

Using this method requires the most patience, but it can net the most profit, as you can loot basically as much of the Underworld in one go as you can carry.

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.

This method can be expanded upon by making "pockets" in the ash that makes up the Underworld. Ash is very easy to mine, so digging lava pathways from the relative comfort of the Underworld's structures can be very useful by guiding lava in the way of mining to a different spot, allowing you to access the entire deposit with less risk. An Obsidian item is still recommended for this method, and even a lava charm for safety, as working directly with hellstone will always be hazardous, but with proper tunnels and platforms, you can cut down a lot of risk while getting a larger yield than just the scraps on the sides of structures.

History
Hellstone Адский камень