There seems to be some manmade marks on the rock, but they aren't very clear. You say it's nearby the head of a spring. A possibility exists that it's a boundary marker of sorts. Perhaps a corner stone.
You mention that this place is close to the Missouri state line. Have you ever heard of a group called the Bald Knobbers who operated in the southwest section of Missouri back around the days of the War of Northern Aggression. They were "night riders" who were known to bully certain "undesirable" folks to leave the area on, literally, a moment's notice. There could well be some unrecovered caches of valuables associated with their activities. I believe the center of their operations was in the Rolla area. There was at least one book published about that group and I bought a copy about 15 years ago. Interesting reading. NO outright mentions of any treasures, but THAT is the kind of book a real treasure hunter uses to investigate "possibilities" because of the situations reported on in the book.