Most likely nodular chert of some type, and not petrified. Looks like it could have some quartz inclusions. It's certainly not a petrified skull.....Petrified bones would look much like ther unpetrified counterparts. If you found a petrified skull you would say "I found a skull," rather than "I found a rock that looks a lot like a skull." Incidently, if the material is chert, it's the most common material that prehistoric people would have used in making stone tools (e.g. arrowheads, spearheads, scrapers, etc).