The injured was roof bolting on the 13 right longwall developement section. He was bolting in the 2nd cut out of the left crosscut. As he let the drill head down he grasped the wrench with his right hand. A rock measuring 23"" x 24"" x 0-2"" thick with sharp edges fell approx. 3' striking his right thumb between the web & 1st knuckle causing a laceration requiring 5 stitches.