Employee stated ee had already installed 2 bolts in row and decided to install a spot bolt next to the rib. After installing the spot bolt, ee swung in and a rock 48"" x 32"" x 4-5"" fell out between bolts hitting ee in the back. EE went to Princeton Community Hosp. ER. X-ray indicated possible fracture of C-7 and ee was given time away from work and referred to specialist