In the 'Grimm' episode "Volcanalis", we were introduced to a demonic humanoid made of magma. The Grimm Wiki has the full details of the nature of this beastie.
The chronicles kept by Nick, which were written by Grimms from the Past, include this entry about the creature:
Volcanalis is thought to have been responsible for the eruption of Mount Vesuvius, which devastated the Roman city of Pompeii in the first century AD.
From the 'Doctor Who' episode "The Fires Of Pompeii", we know that those truly responsible for the eruption of Mt. Vesuvias on that fateful day were aliens known as Pyroviles.
It's pozz'ble, just pozz'ble, that "Volcanalis" was a descendant of those original Pyroviles, who has evolved to a more human as his original rocky exterior became consumed by his inner fires.
It could also be that those rocky exo-skeletons were containment suits holding in the original fiery shape of the Pyroviles, so that they could manually work the terrain. And that means Volcanalis was running around nude.
A splainin like this helps to keep 'Grimm' and 'Doctor Who' in the same "reality" when it comes to the "facts" about Pompeii.