Substance Mimicry (many) – The ability to turn one’s body into several substances.
Absorbing Man (Marvel) can turn into any substance he touches and absorb the substance to become larger (Official Handbook of the Marvel Universe V1 #1).
Amazing Man (DC) can turn his boy into a substance with just a touch of that substance (DC Who’s Who #1).
Metamorpho (DC) is a superhero that has this power.
Nature Elemental – TSR 2158 Monstrous Compendium Annual Volume 2