Take the stone to a large animal vet (horse vet) and ask fir an ID...
Looks like a stomach stone.
They are formed by ground fed animals and may be related to alfalfa hay.
Common to California and SW states...I've seen display cases filled with intestinal stones ranging from small stones to football size. An agricultural University such as Cal-Poly could examine the stone and give an immediate ID.
The stone is made of thin layers of several chemicals common in some forms of hay and soil.
Eventually a semi-football shaped stone is formed that blocks the intestines causing colic, which can cause death.