EE was breaking limestone boulders with 231 Excavator & hammer attachment & had the upper portion of the windshield open. As he was hammering on a rock, a 3""x1"" piece flew at him & hit him in the mouth. It cut his lip open & he had a sore mouth from the impact. He drove to doctor to have it checked out. Dr said teeth ok but put 2 stitches in lip, which made this reportable.