2.8 Billion year old artifact?

This strange sphere was found with hundreds of similar ones by South African miners. They claim the spheres were found in Precambrian sedimentary rock which is 2.8 billion years old. The conventional explanation is that they are some kind of concretion, but they are apparently harder than iron. The three parallel lines around the center are hard to explain by natural processes. Also, these objects appear to be more spherical than most normal concretions.

In what may be a random connection, the spere with the rings brings to mind Saturn's moon Iapetus which has a very unusual equatorial ridge with a similar appearance. More on the spheres can be found here.