The image is of a rectangular piece of marble, lying on a flat pebble beach and with the long side receding away from us. The slab is grooved at the bottom of the side we see in two places. In what appears to be the center of the slab, a large round dome of glass is partially inset. If we approach more closely, we see that the glass is placed over and partially within a circular hole pierced through the solid marble. The glass magnifies whatever is beneath it, in this case only tiny pebbles.