Employee stated after T bar was set, EE walked forward to swing boom outward to install EE's rib bolt. A piece of draw rock 6'L X 3'W X 2""T fell on EE's back forcing EE to the ground. Once EE caught self with EE's hands, the rock slid down EE's back striking EE's right hand causing bruising. Employee was transported to doctor for X-rays which were negative for any fractures.