Obsidian Skull

The Obsidian Skull is an accessory that grants immunity to fire blocks, i.e. Meteorite and Hellstone, but not lava or fire-based attacks. It also provides a modest stacking defense bonus of 1 while equipped. This stacks with the 1 defense per Cobalt Shield and 1 defense per Shackle.

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