Another Weird Rock

I was on my neighbors property looking for rocks in the creek that is just upstream of the creek on our property.   After I was done, he gave me the round rock below and asked if I could cut it.   Always up for a challenge, I said sure.   I have a geode clamp that I have never used because it wasn't constructed very well and you have to very exactly place it so the saw isn't cutting through the clamp.  Anyway I mangaged to get it clamped and cut with only a little rubbing of the clamp on the saw blade.   I think I am going to have to file down part of the clamp if I use it again.











When I first looked at the rock, I thought it might be a Native American grinding rock (because it was so round and no geodes in this area) and maybe granite or quartzite.  You can't tell from the cut photos but I think the rock is basically a limestone concretion (there are small fossils in the matrix including crinoids).   So I don't think it would be an artifact because not very hard and wouldn't stand up to grinding.  In fact, the hollow portion on the inside was full of "sand" when I cut it open.  It washed out very easily. 

When I lived in Michigan, I found a lot of concretions but they were made from a very fine grained silty looking material, not larger sand particles like this.  Does anyone have any other thoughts on what this might be? 

Thanks for looking,

Chris