On PRIVATE property ? You can do whatever the heck you want on private property. You and farmer-Bob can keep or collect or whatever-you-want.
Perhaps in some countries , like England: All resources under the ground belong to the queen. But here in the USA, what's on your land is outside the laws pertaining to public land. If you strike oil on your land, you're rich. If you find a goodie, it's yours. I know of no law in ANY state that says if you find something (even indian related) that it's not anyone's business but your own.
