Kie-Pur, you naive innocent fool ...

What does he mean, “he wasn’t suppose to use ground magic”?

Remember?
He is a fire mage. To perform ground magic he has to use the ground Katarak crystal. And magma is heated stone.

Therefore ...there is something wrong, or is it?

Like i said, to heat the stone, you need fire, so it's somehow related.

Or, another kind of energy.
btw. I fixed the graphic for the Katarak Crystals. The missing lines were added.