Yes, rock.  These are chunky, clunky, craggy rocks, not stones.  And this one is different from all the others surrounding it.  What a complex and unusual surface it has: it looks a bit like an archaeological excavation site.  There is also something bone-like about it, with the white ridges and holes and skeletal-looking shapes.  Anomalous indeed.  I’ve walked by this area dozens of times and never noticed it, but the sun must have hit it just right to grab my attention!