I'm no rock expert, but I think it's limestone. Whatever the white rocks that are all over central Texas on the top of the ground. This one is obviously darker from fire and dirt.
David the best way to tell is to get a hammer or something and break the rock. If it is fire rock you will have a bad smell just as you break it. That is from the methane gas trapped in the rock when it was super heated. No Bull !!!
Bill Arnold