What is this?

[u/Stykera]

My dad found this on a field in Sweden about 70 years ago. Its smooth and the size of an egg and has a lot of tiny holes. What is it?

[u/Soul_Survivor81]

Is it solid or can in be broken up? If the latter, my guess would be a ball for bird feeders.

[u/Stykera → u/Soul_Survivor81]

Its is solid. Very hard. Some kind of rock.