Molten Charm

You get a timer, not permanent safety. Touch lava and a small meter appears over your head and starts draining, exactly like your breath does underwater, and it refills only after you climb out and much more slowly than breath does. Treat it as a window for crossing a lava lake or grabbing an ore vein, never as a licence to live down there.

The second half of the accessory is the part people forget: it stops the burn you take from standing on meteorite or hellstone. That is the effect that makes Hellstone mining bearable. It does not protect you from fire damage in general, so on-fire debuffs and burning enemies still hurt normally.

Every other lava immunity accessory adds its own window on top of this one rather than overlapping with it, and a certain minecart contributes as well. Wearing two or three of them stacks up enough time to strip mine an entire hellstone layer without surfacing. Nothing new is unlocked by doubling up, only more seconds on the meter, so past your first couple the slots are usually better spent elsewhere.