The Key of Magma (マグマの石, Maguma no Ishi?, lit. Magma Stone), also called Magma Stone, is an item in Final Fantasy IV which opens the way to the Underworld, where the Dark Crystals lie.
Kain gives the Key of Magma to Cecil after being temporarily freed from possession by Golbez. Kain joins the party and Cecil tosses the key down a well in Agart to open the way into the Underworld.
Spoilers end here.
The Magma Stone is a dummied out item, only accessible via hacking.
Magma is a mixture of molten or semi-molten rock, volatiles and solids that is found beneath the surface of the Earth, and is expected to exist on other terrestrial planets.
The name is a reference to the Magma Staff from Dragon Quest IV.