Mining Hellstone can be a pain, not only because Hellstone deals fire damage to the player (which can be negated with an Obsidian Skull); but mainly because of the dangerous monsters of The Underworld, as well as the fact that Hellstone spawns lava (half a block per block of Hellstone ore) when mined.
One approach to mining Hellstone can be to just drop down to the Hellstone/Ash "floor" and start digging out the Hellstone. One must keep in mind, however, that Hellstone Bars are very expensive and difficult to acquire as 3 pieces of Hellstone and 1 piece of Obsidian are required to forge only one bar.
A second strategy is to build a little box around the hellstone you would like to mine with a low tier block and then simply mine inside of that box staying as close as you can to the box so that you do not miss any of the mined hellstone
A third method requires several Obsidian Skin Potions. You may also want to use a Cloud in a Bottle, Rocket/Spectre Boots, Flippers, or a Grappling Hook. Find a pool of lava, use the potions and jump in. They are usually large. Simply use the Grappling Hook to stick close to the Hellstone while you mine it, escaping using your preferred method when your potion supply runs out. This technique will allow you to collect a decent amount of Hellstone, depending on how many potions you have. The forth method is to dig all around the hellstone leaving one layer of any block between you and the hellstone. Then you can grapple on to the block and mine it through the block and still pick the hellstone up.
Dealing with the Hellstone itself
As for dealing with the lava that is spawned when Hellstone is mined, you can use an Obsidian Skin Potion to negate damage from lava, as well as preparing a lava barrier to keep the lava from spilling onto you, and simply mine the Hellstone from behind the barrier. Prior to Update 1.1, you could also safely mine Hellstone by placing blocks of sand to soak up the lava formed as the Hellstone blocks broke. Another way is after mining 2, use a bucket and take the lava out.
- Hellstone will not spawn lava when mined if it is located outside of the Underworld.
- Hellstone does not hurt NPCs when they touch it. For some reason.
- Hellstone was added after a user suggested a "firestone" of sorts. 
- Changed the recipe from 4 Hellstone and 1 Obsidian to 3 Hellstone and 1 Obsidian.
- Max stack increased from 250 to 999.
- Can be destroyed by explosions once the Wall of Flesh has been killed.
- Now gives off fiery sparks (Graphical effect only).
- Spawns lava when mined, but only when mined in the Underworld.
- Added to the game.
|Original Metals||Alternative Metals|
|Hard Mode Metals|
|Other Minerals & Materials|
|Items Requiring Hellstone|
|Tools||Molten Pickaxe · Molten Hamaxe|
|Weapons||Fiery Greatsword · Flamarang · Hellfire Arrow · Imp Staff · Molten Fury · Phoenix Blaster|
|Armor||Molten Helmet · Molten Breastplate · Molten Greaves|
|Potions||Flask of Fire|
|Other||Hellstone Bar · Hellstone Brick|