By all scientific accounts, theses rounded objects found in pyrophyllite deposits around Ottosdal, South Africa are three billion-year-old rocks that were naturally formed by carbonate concretions.
The tiny pyrophyllite spheres ranging in size from .5-10 cm, with even latitudinal grooves on them, were created when minerals formed in the space between sediments much they way crystals can naturally form extremely precise shapes. It was the weathering of these specimen that then left them as tiny balls, with evenly spaced lines circumscribing them.
Of course, this answer does little satisfy those who are convinced the spheres are too perfect to have been created naturally. Since their first discovery in South African mines, they have been a hot topic among conspiracy theorists who see them as proof of ancient aliens.
Much of the doubt about the origin of the spheres originated with an unverified story in which a man brought one of the spheres to NASA. According to this tale, after NASA tested the stone, they told the man that the sphere could have only been made in zero-gravity because its balance was too perfect to have been created naturally. There is of course no documentation of this story, and close examination of the spheres have disproved any claims of "perfect balance," along with the claims that the spheres are "harder than steel."
Nonetheless the spheres serve as a jumping off point for pseudo-archeological claims that intelligent beings roamed the earth the billion years ago, and that these beings made the spheres for religious or military purposes. Some even claim the spheres were an ancient form of information technology made by and ancient people millions of years before the first men and women walked the earth.
Despite these spurious claims, the scientific world agrees that the spheres are simply a natural wonder of the world, to be appreciated in their own right.