I wonder if it may be related to a bullaun? Or something like the "Plague Stone" outside Zennor museum.
Good find! Wonder what other people think..?
G x
From what I can remember, it's near-as-dammit a perfect circle, and while the depression feels comparatively smooth / regular to the touch, it's got the same rough basic texture as the rest of the stone (if that makes sense?) - it's also fairly shallow, so I'm guessing it's not been scoured out by a stone and water, nor do there appear to be other marks of any kind on the stone.
I'll go and have another look next time I'm on the island, and will also definitely try to take in Rubha Charnain as well - had to give it a miss this time as we were "doing Harris" in a day before getting back to Callanish in time for for the moon skim!!!