It is often imagined that if Slugma sleep somewhere away from the hot magma vents or the Earth's mantle, both of which many call home, they will harden into a petrified Slugma-shaped statue. This is not exactly the case; a Slugma will harden into rock, but will evolve in doing so, becoming a Magcargo. And yet Slugma resist this evolution with almost as much raw determination as most pokemon resist death – a likely source for this common misconception.
Slugma are probably the oldest pokemon known to science, an anaerobic relic of the Precambrian age. They have had only one significant change since that bygone era, one which allowed them to survive in the oxygen-laden atmosphere of the modern world: still, they prefer sulfur for respiration, and are therefore usually found in locations where sulfur is common enough to survive on. The reason their genome is so resistant to change is probably because of Slugma's own staunch conservatism - many refuse to mate with their shiny, gray counterparts, let alone other species in their egg group – and this fear of change applies equally well to an individual's evolution as to the species as a whole.
Or perhaps this explanation too is a red herring, and the true answer is a far simpler one. Magcargo, after all, burn hotter than the Sun, and most larval Slugma, even with their enormous tolerance for heat (being made of magma themselves) have been accidentally scorched by their mothers soon after hatching, for Magcargo are prized as incubators. Although Magcargo themselves do not burn in the manner of Magmar, few Slugma realize this, and none wish, like a pyromaniac Midas, to make everything they touch turn to ashes.
