What is "petrosphere"

n. (context archaeology English) Any roughly spherical artefact made of stone

In archaeology, a petrosphere (from Greek πέτρα (petra), "stone", and σφαῖρα (sphaira), "ball") is the name for any spherical man-made object of any size that is composed of stone. These mainly prehistoric artifacts may have been created and/or selected, but altered in some way to perform their specific function, including carving and painting.

Several classes of petrospheres exist, such as the stone spheres of Costa Rica; painted pebbles from Scotland; stone charms from Scotland; sandstone balls from such sites as Traprain Law; the Carved Stone Balls, which are mainly from Scotland, although they have also been found in Cumbria and Ireland; and carved stone shot for cannons and trebuchets.

Naturally formed stone balls, such as concretions and spherulites, have been at times misidentified as petrospheres. 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, found around Cerro Piedras Bola in the Sierra de Ameca, between Ahualulco de Mercado and Ameca, in Jalisco, Mexico, are petrospheres. 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, they are composed of granite, and natural processes cannot produce stone balls. Similarly, cannonball concretions, i.e. those found along the Cannonball River in North Dakota and near Moeraki, South Island, New Zealand, also have been misidentified as petrospheres.