Obsidian Skull

The Obsidian Skull is an accessory that grants immunity to fire blocks, i.e. Hellstone and Meteorite, but not lava or fire-based attacks.

It is important to note though, that it is not essential to have one equipped while mining Hellstone or Meteorite as you only take damage when touching the minerals. As such, if you stand at a distance you can still mine the ore safely.