A partially polished ammonite fossil.
At one point in history, these guys ruled the world. A few million years ago, there were bajillions of varieties of nearly every size and shape imaginable… as long as what you’re imagining has a spiral in the middle. Although they look a lot like the chambered nautilus, their closest living relative is the cuttlefish.
Medieval Europeans called ammonoid fossils “snakestones,” and believed they were proof of the snake-exterminating miracles of Saint Patrick and Saint Hilda. In India, the swirling stones were seen as traces of the god Vishnu.
Something about spirals in the middle, I think.