I too have found rocks that look a lot like that one- but a different lithic. Still- with dimples and indentations on one side and not on the other. If you haven't already, try looking at those impressions through the best magnifier you have- and see if they're impressions of little pre-historic critters. I thought it was really onto something until I looked really close. In my case, what I found were clearly once-living 'things' (for lack of a better word- I don't know the names of fossils or fossil impressions). Got to say, though- all the mortars and paint cups and nutting stones I've ever found were not smooth like that. They have the rough look of use and wear- not slick/wet/'oily'-looking. To me, it looks like sedimentary rock with impressions of long-dead wet-world creatures.