What is another word for magma?

Magma doesn't have an antonym, in that there is no Word or expression that means its direct opposite.

That being said, magma on the surface (as opposed to underground) is called lava; and once it cools, it's called lava rock or igneous rock.

The other parts of the planet (i.e. not the magma) are the crust and the core. Perhaps one of these terms is what you were looking for.