Employee stated that EE was in the process of removing rock off of the left C/M Machine on the Unit 2 Section. EE stated that EE was going to attempt to push a rock off the machine so they could move it. In the process of EE pushing the rock off the C/M machine it broke. This caused EE's hand to slip allowing EE's arm to strike the sharp edge of the rock. EE received 6 stitches.