Four hundred and twenty geometric stone spheres have been found in the vicinity of Neolithic stone circles in Northern Scotland, with 169 coming from Aberdeenshire alone. Outside Scotland, examples have been found in Ireland at Ballymena, and in England at Durham, Cumbria, Lowick and Bridlington. One was recently spotted by the author over 6,000 miles away that came from an important megalithic pyramid site in South America. What are these strange objects and what were they used for? Hugh Newman investigates.