Employee was operating #5 stinger ride on #1 unit traveling outby the last open in the #5 entry. The employee drove thru a line curtain and was struck by a rock. The rock was approximately 4'W X 5'L ranging from 1/2"" to 5 1/2"" thick. No indicators of adverse roof and roof support was installed IAW approved roof control plan. The rock fell out between middle roof bolts.