Hot to the touch
Hellstone Bars are made from 1 Obsidian and 3 Hellstone at a Hellforge. They are used to create Hellstone Tier items. Items made from Hellstone Bars are the highest quality craftable pre- hardmode items that require bars.
Terraria Hellstone Tutorial 1.1.2
You can use an Obsidian Skin Potion and mine Hellstone under Lava without getting damaged.
To craft the entire set of Hellstone tier items it takes 140 Hellstone Bars, which equals 140 Obsidian and 420 Hellstone.
When you mine Hellstone, it leaves half a block of Lava behind it.
A challenging but worth-the-work way to get the obsidian for these items is to drain your ocean down to the Underworld.
Recipe changed: Now requires only 3 Hellstone, instead of 4.
Recipe changed: Now requires only 4 Hellstone, previously required 6.