[Here are 2 articles with lots of photos. This relates to the mystery of wonderfully smooth carved stones that seem to serve no purpose, as well as stones that are very artistically carved. It seems to me our brethren were doing these things. The purpose might simply be art. There are lots of photos in the articles below. Jan]

Here are 2 articles about these:

https://www.livescience.com/62845-scotland-carved-stone-balls.html

https://www.livescience.com/mysterious-stone-balls-scottish-tomb-orkney