PERSON WAS ADJUSTING BELT STRUCTURE TO TRAIN BELT OVER AT THE LONGWALL TAILPIECE. A PIECE OF ROCK 8"" LONG X 6"" WIDE X 1 1/2"" THICK CAME OFF BELT AND STRUCK HIM BELOW THE LEFT KNEE. PERSON WAS WEARING KNEE PADS BUT HIS LEG WAS PUSHED BACK AT THE KNEE, CAUSING INJURY.