Musar על חגיגה 28:8

When he had said "the G–d of my father has come to my assistance" (Exodus 18,4), he implied that even his father Amram had become so refined and purified that he qualified as an איש האלוקים. The words "He has saved me from the sword of Pharaoh," in addition to "the G–d of my father came to my help," mean that just as one grants additional wisdom to those who already possess a degree of wisdom, so Moses, seeing he had already possessed basic qualifications, advanced to even higher rank both spiritually and physically, so that even Pharaoh's sword could not harm his neck anymore than it could harm a pillar of marble. This is a hint that marble is incapable of becoming ritually impure. When the four sages entered the פרדס, i.e. the study of esoterics, מעשה מרכבה, Rabbi Akiva warned his colleagues "when you see pure marble, do not say "water, water!" (Chagigah 14)