Hard to say. Kinda resembles a plummet in the making, with the knob finished, but I've never seen the like before, so that's just a wild guess suggestion.
Its pretty much the same all the way around.It looks unfinished but i suppose it could have been used as is but i dont feel it.Larger than any plummet ive ever found but i do believe it was to be a weight.
No offense but it looks like a piece of natural limestone like what I find in the creeks here. If it is and was worked it would be smooth and show some discoloration where it was worked. I see no difference in the color of it any place on the piece. I believe it is natural, sorry.
I have seen a lot of similar rocks in good numbers, with that feature on the top. It looks odd, and I have kept a couple, but the more I find I believe it is just a natural occurrence.