Magma Ball

The Magma Ball is a Rare item gained at Magma Rock. Near the end of the dungeon, around the time you gain the Blaze Psynergy, there is a room with lava and platforms and it "erupts", spewing out one particular chunk of hot rock in particular. Go to the rock and take the Magma Ball item from it. It can be used with the cannon to break down the wall in Loho, and therefore prompt the dwarves to install the cannon onto your ship. Then at the entrance to the Northern Reaches, the Magma Ball and cannon are used to break the wall of ice and allow you to enter the game's final region, containing Prox and Mercury Lighthouse.