In Argentina, in el Valle de la Luna, we have a place with a lot of big natural stone spheres. They are bigger than the sphere in the picture, so perhaps it has a different geological origin.
http://commons.wikimedia.org/wiki/File:Valle_de_ichigualasto...More info about natural stone spheres: http://en.wikipedia.org/wiki/Stone_ball#Natural
Some pages with photos of similar places:
http://101lugaresincreibles.com/2010/10/cinco-sitios-con-roc...
http://www.cesarlizano.name/sibowak/esferas_de_piedra_costa_...
