Iron

If you're higher than that, there's a tricky part where you go through a cave archway and then climb the NE ladder to a red crystal, go back down to a room that's mostly magma, climb down a ladder to the level with all the grunts and the redhaired scientist (when it's neutral), then surf across some magma in the SE part. That will lead you to a big blue crystal, and it's pretty easy from there.

Try using Rock Smash on the pillar/termite mound looking thing. That was what I ended up needing to do when I was stuck. Then you can go north over the magma.