If the ball is limestone, then I would lean toward water filtration or some form of filtration. Limestone is soft and porous.
A sphere has the largest surface area per unit volume. When layered in a bed or stacked in a tube/tower, you get excellent filtration without impacting media flow.
If the ball is a harder stone, then milling isn’t out of the question.
Fun find! It seems lonely, you need more. 😉