see if a magnet sticks to it(really sticks...like too much)if so...perhaps meteorite.Arizona world famous for em.Also if meteorite prolly elemental iron content...super heavy.
if non magnetic...my next best guess would be a geode.a rock withe a crysteline center.Take it to a rock store and have em cut it in half for you.Its not the mona lisa and cut properly could(likely)provide you with a real beauty