Note: See webmaster's comments following description.
"Over the past several decades, South African miners have found hundreds of metallic spheres, at least one of which has three parallel grooves running around its equator. The spheres are of two types--'one of solid bluish metal with white flecks, and another which is a hollow ball filled with a white spongy center'.
Roelf Marx, curator of the museum of Klerksdorp, South Africa, where some of the spheres are housed, said: 'The spheres are a complete mystery. They look man-made, yet at the time in Earth's history when they came to rest in this rock no intelligent life existed. They're nothing like I have ever seen before'.
We wrote to Roelf Marx for further information about the spheres. He replied in a letter dated September 12, 1984: 'There is nothing scientific published about the globes, but the facts are: They are found in pyrophyllite, which is mined near the little town of Ottosdal in the Western Transvaal. This pyrophyllite (Al2Si4O10(OH)2) is a quite soft secondary mineral with a count of only 3 on the Mohs' scale and was formed by sedimentation about 2.8 billion years ago. On the other hand the globes, which have a fibrous structure on the inside with a shell around it, are very hard and cannot be scratched, even by steel.' The Mohs' scale of hardness is named after Friedrich Mohs, who chose ten minerals as references points for comparative hardness, with talc the softest and diamond the hardest. 
Mystery Spheres Stump California Space Institute?
Stones, which are found in rock scientists say are billions of years old- and which rotate on their axes, captured the attention of Mr. John Hund of Pietersburg fifteen years ago....
While playing with the stone on a very flat surface at a restaurant one day, Hund realized it was very well balanced. He took it to the California Space Institute at the University of California to have tests done to determine just how well balanced it was. "It turned out that the balance is so fine, it exceeded the limit of their measuring technology and these are the guys who make gyrocompasses for NASA.
The stone is balanced to within one-hundred thousandths of an inch from absolute perfection," explains Hund. Nobody knows what these stones are.
One NASA scientist reportedly told Hund that they do not have the technology to create anything as finely balanced as this. He said the only way that either nature or human technology could create something so finely balanced would be in zero gravity."
Notes from the Webmaster:
As there usually is when it comes to OOP artifacts, there is some controversy over the Klerksdorp Spheres. Most skeptics bash Cremo on his supposed source (a weekly world article), rather than address the object itself and conveniently ignore the fact they were mentioned a great deal earlier than Cremo's 1993 book. For example:
For a good counter-argument, see the Wikipedia article on the Klerksdorp Spheres. In summary, the main arguments against them are:
However, at this point, it has yet to be proven conclusively either way as to whether these are legitimate OOP artifacts or not. Until these can be firmly debunked, I will leave with the other artifacts. You the reader must decide for yourself whether or not you think these are man-made.
 Jimison (1982)
 "Forbidden Archaeology", M. Cremo and R. Thompson (1998)