To mine Hellstone in Terraria, you need at least a Nightmare Pickaxe or a Deathbringer Pickaxe. These are the only pre-Hardmode pickaxes that can mine Hellstone. The Molten Pickaxe is the strongest pickaxe you can make before entering Hardmode, and it is the only one that can mine the initial ores that Hardmode introduces. However, to make the Molten Pickaxe, you need Hellstone Bars, which require Hellstone Ore, Obsidian, and a Hellforge to make. This means youll need access to the Underworld and at least a Nightmare or Deathbringer Pickaxe before you can grab the un-craftable Hellforge from the Underworld dungeons.

In summary, the minimum pickaxe you need to mine Hellstone is either a Nightmare Pickaxe or a Deathbringer Pickaxe. If you want to make the Molten Pickaxe, you need to mine Hellstone with one of these pickaxes and then use the Hellstone Ore to make Hellstone Bars, which can be used to craft the Molten Pickaxe.