What is it??

 

My first thoughts were of a mortar and pestle but I think it is probably just a rock that had a natural depression where another rock fit into and the forces of wind water and time eroded it right through. It's to clean to have been worked that smooth by hand.

I'm pretty sure it's an artifact, Hawk.

May have originally been rougher smooth & been smoothed by the water & sand.

Nice find

Seems a mighty big coincidence to be worn so uniform in shape by nature. 
If an artifact, could it have worn so smooth over the years in the river?

Abking, is there a professor from your anthropology days you might ask?
I haven't heard from the Grace Hudson Museum yet, and don't know if they even know.

There must be quite a few nor cal native artifact experts we could ask.
I'm just really curious.  -Matt

May have originally been rougher smooth & been smoothed by the water & sand.

My thoughts also I find a lot of arrowheads and I found 1 site where they were making them and the further down river I find them the smoother they are. It might have been worn by use also.

I'm no archeologist but I did stay at a Holiday Inn

Seriously though I did spend my youth after the service living a hunter gatherer lifestyle. This is a picture of the implements I used everyday. Notice the pump drill in the center. It's cedar wood with a tip made of Quartzite. For a counter weight I and many other natives across the millennium used a donut shape weight made from clay. It looks exactly like Mikes rock there. I can't imagine the natives working stone like that with the abundance of pottery used here on the west coast. I'd almost put money on it that it's natural.

  My initial thought was it was a macro shot of  Cherios cereal!  after that, maybe an anchor of some sorts.

....it belongs to Mahi Mahi.....it must have fallen out of his patrol car....the healthier peace officers of Ukiah seem to like the plain doughnuts better then the chocolate covered ones ...according to my research

I think Swellrider is on the right track, but it might not be indian.....possibly european visitors/traders?..... who knows? ,cool find tho.....
Google "spindle whorls"
Tony

....it belongs to Mahi Mahi.....it must have fallen out of his patrol car....the healthier peace officers of Ukiah seem to like the plain doughnuts better then the chocolate covered ones ...according to my research

I agree Joel but this one looks to be the brain variety...