Is the Devil's Face on the Dome of the Rock?

A friend sent me these pictures of the Dome of the Rock--Islam's third holiest site. He pointed out the creepy image on the outside of the building, which some claim to be the face of Satan or some other demon. I'm inclined to see this as a random pattern produced by natural rock formation, but I have to wonder why Muslims chose a slab of marble that has an image that does look rather scary (horns, eyes, a nose, a jagged mouth, flames around the head, a neck, etc.). Do any of my Muslim friends have an idea of why the builders didn't use a different piece of rock? (You can see the location of the slab at the bottom of the second picture.)