Stone ball

These mainly prehistoric artifacts may have been created or selected, but altered in some way to perform their specific function, including carving and painting.

For example, fringe archaeologists and advocates of prehistoric extraterrestrial visitors have repeatedly argued that the stone balls, which range in diameter from 0.61 to 3.35 m (2 ft 0 in to 11 ft 0 in), found around Cerro Piedras Bola in the Sierra de Ameca, between Ahualulco de Mercado and Ameca, in Jalisco, Mexico, are petrospheres[citation needed].

However, these natural stone balls are megaspherulites that have been released by erosion from a 20- to 30-million-year-old ash flow tuff, which originally enclosed them and in which they formed.

The proponents of these stone balls being petrospheres base their arguments on the false claims that all of these spheres are perfectly round, that they are composed of granite, and that natural processes cannot produce stone balls[citation needed].

Some archaeologists argue that they were deliberately shaped by humans to use as tools; others that they are byproducts of the use of rocks for other purposes.