MN2:30 seems to identify the stones of pure marble with "water that above the firmament" (Gen. 1) which he says is no water but "higher matter". (He then indeed makes metaphysics serve as physics, following his day's philosophy that was hostage of his day's lack of physics but I don't think this matters.) So (his point is) it would be falsehood to call it water. So his view ("don't mistake more perfect [marble] for less perfect [water]") seems contrary to yours ("don't mistake less perfect [marble] for more perfect [water]", which is also "the Rebbe's" view]. Unless I miss your/his/Rebbe's points, which is quite possible.