What was the weather like around there? Any chance that a pebble had been picked up by a twister some distance away and then fell when it got spun out of the high wind area? Now, if you know a physicist, and if you found that pebble and kept it, (s)he could maybe calculate from the (lack of) damage to your helmet what the maximum speed might have been that the pebble had when it hit your helmet, or perhaps some range of speed. From that, you could also calculate the height from which it could have fallen, which can make all the difference between, say, something dropped by a bird from up in a tree or something spinning off the wheel of an airplane just tucking it's landing gear in or out, or something dropped by a high wind.