Google Streetview (don’t worry, I’m not using that to map, I know they’re there and they can be seen in Bing aerial imagery, link is just for convenience to enable discussion).
They’re designed more as sculptures rather than barriers. So I don’t think barrier=block or natural=stone would be correct.
I was leaning towards tourism=artwork + artwork_type=sculpture but that doesn’t quite seem fitting - certainly not for each individual “ball”. But perhaps that is right?
Is there a way to tag their general spherical nature?
These appear to be pre-cast concrete with a polished finish; in North America they are typically sold as decorative bollards. They may have been chosen for this public square because they look more ‘interesting’, but they’re (usually) factory-made: they’re not sculptures, and I wouldn’t consider them ‘art’.
To me they look like they’re meant to provide (very modest) physical separation between pedestrians standing around and admiring the fountain, and other pedestrians/cyclists/passers-by who are just travelling through the square.
If you want to use an existing tag, I’d day these are barrier=block. A description could be added that these are balls made of stone or concrete, whatever applies.
The whole Victoria Square is designed as an artwork withouth doubt, described in detail here
but the stone balls apparently are not a real part of the art elements. Judged by the aerial imagery I strongly believe they are in fact placed there as a barrier although being nicely shaped to fit into this place.
You won’t find out unless you ask Birmingham city administration. As long as they don’t confirm these stone balls are just for decoration I would still see them as barrier although being within the pedestrian area.
I don’t think they are barriers. What would they be a barrier to and to whom? They’re placed well inside the pedestrian zone which is already blocked off for vehicles through bollards (Google StreetView is better for seeing this). Certainly they don’t stop pedestrians.
Several references online refer to them as sculptures though I can’t find anything “official”. Edit: but I do note that they are not listed in the artwork section of the Victoria Square Wiki page.