The question is...
...what were they? Did they perform a work-related task? We're they religious markers?
The ability to shape them so perfectly and move them in an ancient setting seems like it would require a herculean task of laborers. It would be difficult to shape such rocks today, nevermind hundreds or thousands of years ago. This implies then that the objects were highly valued.
So something highly valued brings several potential uses:
Currency (very hard to steal currency).
Religious symbol.
Property line markers
Commemorative objects (like Runestones)
Spherical tombstones
Then again, they may have been shaped by some unknown force...