While installing roof bolts in the x-cut 5-6 room, the employee installed a rib bolt and then swung in to install the middle bolt. When he swung the bolter head in, a piece of stone approx. 24""x12""x1"" fell from between his last bolt and the rib and struck him on the calf of the right leg. The accident resulted in a non-displaced fracture to his right fibula.