Employee was roofbolting in Unit #2 in Entry #2. He walked to the back of the roofbolter to make up a roofbolt when a rock fell out between the rib pin and the center pin. The rock was approx. 2'x2'x2"". The rock struck him on the left side of his neck and then slid down his left arm. He received a laceration to the left side of his neck requiring sutures.