The Klerksdorp Spheres are strange objects found and collected in South Africa's 2.7-3.0 billion years mineral deposits by miners. They are between 5 and 100 mm in diameter and have variable spherical or disc shape. They can have also grooves on the equator. They are often claimed to be evidence of technology of very prehistoric non-human civilizations, remainder of extraterrestrials or similar believes. Below I am listing some sources with pictures and different explanations.
Sources: Virtuescience, OOPArt, The TalkOrigins (skeptical point of view).