Is it a fossil? Is it a rock? Is it an eye?
No! It's a gastrolith also known as a stomach ossicle, a stomach stone, or a crab's eye.
They are produced inside freshwater crayfish and are sometime found around dams and swamps. As animals such as crayfish, insects and spiders (Arthropods) grow, their old rigid skin becomes too small and they need to moult it to make more room. The new skin underneath is soft, allowing them to swell to their new larger size. Then it hardens quickly to protect them from enemies. In crayfish the skin is strengthened with minerals particularly calcium carbonate. As fresh water is very low in minerals, crayfish need to absorb calcium carbonate from their old skin before moulting. It is stored it in the stomach as a gastrolith and reabsorbed after moulting to harden the new shell. Gastroliths are solid hemispherical objects that are resistant to weathering and survive long after all other parts of a freshwater crayfish have decayed.
In the past, in Europe, they were known as "crabs' eyes" or "crabs' stones" and were used in medicine to settle stomach problems. It would have been an effective antacid treatment and calcium carbonate is still used for this purpose.