Even stuff that is impervious to fire when placed, like chests, diamond blocks, etc.
If you die in lava then there's a chance that some of the stuff is on the edge, if you haven't been back already and looked.
If you manage to climb out of the lava and die from being on fire then there's a pretty good chance that at least some of the stuff survived.
(It's been so long I can't actually remember if some stuff is burnt up if you die from being on fire but I think so, I can remember being on fire and thinking I should keep moving to spread my stuff out if I died.)
Unless you're on a server with thoroughly explored oceans it should be pretty easy to find another Heart of the Sea, it's the Nautilus Shells that take the most work to replace.