It looks like it might be river stone that was baked by the prehistoric people , intentionally or not , as they comonly did , looks like heat and moisture made it pop .
1st - I noticed this was your very first post chris1211 - so, Welcome Aboard! You didn't list your state (or country) in your profile. So, you might consider jumping over to Sub-Forums: Select Your Area.... and selecting location information (i.e., clubs, hunts, finds, legends, maps, etc.) directly related to your state (or country).
2nd - I moved this thread from Today's Finds! over to Forum: What Is It? for more exposure.