EMPLOYEE WAS WORKING ON SURFACE REPAIRING REFUSE BELT. HAD LOWERED TOOLS TO GROUND LEVELS AFTER REPAIRS COMPLETED. WAS IN PROCESS OF PICKING UP TOOLS, A PIECE OF SLATE APPROX 6 INCHES IN DIAME TER AND 2 INCHES THICK FELL FROM REFUSE BELT - APPROX 20 FEET - STRIKING EMPLOYEE ON CENTER OF HARD HAT - COMPLAINED PAIN TO NECK.