I found this ball in Rovereto, Italy. It is precisely spherical, and thus clearly manmade; with the exception of a few very light scratches on the surface, it is completely featureless. It also appears to be a single piece of solid marble of roughly 2.5cm of diameter.
The sphere was laying among small rocks and gravel in the middle of a path in the forest that runs in an ample circle from the Ossuary of Castel Dante to Maria Dolens, the bell forged from weapons of the first World War. While close to the town in terms of absolute distance, the path is deep in the woods and isolated from houses and settlements in general.
Although it is clear that someone lost it, I have not been able to find a satisfying explanation of what this exactly. I now keep it as a fidget of sorts, and as souvenir of that amazing trip. If you have a convincing answer or evidence, contact me and you will win a free beer.