How bizarre! Prepare for my "Encyclopedia Brown" investigation explanation. I am not an expert or any more knowledgeable than anyone else-- I am a layman with access to too much information. Please take the following with a grain of salt.
Apparently this isn't an isolated phenomena. Stone spheres of all different sizes and materials have been found in many different places such as China, Egypt, New Zealand, Easter Island, Costa Rica and more, and seem to be used in a number of differing ways with no concise use or answer.
It's considered possible that these stones acted as land marks or even astrological marks, representing either certain consolations, settlements, significant or certain areas, or just direction.
It's as well speculated that these spheres represented something more personal and religious, having been found in burial mounds, they could represent some form of respect towards any number of gods or entities.
What I find telling though, after doing some research, while the people that created these curiosities were living in the stone ages, it seems apparent that some of these societies (more-so than others) were actually pretty advanced for their time, and it is speculated that possibly these stones signify this intelligence, or its advancement in life and society.
Costa Rica's Diquís people, for example, which created some of the most defined and well kept stone spheres, inhabited different chiefdom settlements (differing from the chief of a tribe or band, chiefdom describes a complex societal organization seen to be a more primal development of the concept of a "state") that were known for their artistry. Not only did this civilization embody a more complex society, and create these spheres from supposedly nothing but rock tools and sand, they as well created special gravestones and stone cists (coffin), walls, monoliths, roads, fountains, figurines, vases, jewelry, body art, and clothing out of many varieties of materials (most notably gold) all of which ranged in complexity and signifying influences. They as well practiced hunter gathering, slash-burn agriculture, and even trading with other distant land masses.
Personally I would like to believe that these stone spheres, like the development of style in art, signified a development in people, perspective, and society that ultimately became popularized through trade before being lost and forgotten.