http://www.themodernantiquarian.com/site/4299

http://www.themodernantiquarian.com/site/3572

http://www.themodernantiquarian.com/site/8051

http://www.themodernantiquarian.com/site/7284

this is a disputed rocking stone:

http://www.themodernantiquarian.com/site/108

As far as I know a rocking stone is a stone that rocks, sometimes called a logan stone. It's a category of site at the modern antiquarian.

With respect, all stones are natural, though some, because of where and how they have been placed, manipulated and/or worked, are deemed to be interesting by some.