Like Salva said-- do not platform to anywhere else, just stick to the east edge. Past the giant whale(?) bones and the three cuttlefish.

Okay so, the item IS there but for some reason the lava and the ground look exactly similar in the location where you find the item. It makes it stupidly hard to find. But it IS there.

It's really there, just go east with the lava to your left side. You don't need to jump or platform, just follow the edge of the lava while going straight up east.

Having a LOT of trouble finding this cave...