I can give you a few guidelines, but you'll still have free reins to do what you want with it!

Basically, it's true that it's not a rock. However, that's because it's a creature that is made of rocks. Think of it like an old fashioned elemental rock golem of sorts, only this one can take on forms. It prefers snake-like appearances, sometimes even going as a basilisk (so, you know, to "stone" people, hah!).

This means that its body is extremely tough, and it will utilize its surroundings to camouflage itself either amongst other large rocks/desert cliffs or even burying most of its body under the sand with its head jutting out like a boulder.