This Egg Group is primarily composed of Pokémon sharing a distinct characteristic: they are of an inorganic nature, many taking on a rocky, crystalline, or metallic appearance. In some cases, they either resemble man-made objects, possess man-made objects, or are man-made (by accident or on purpose). A little under half of this group are gender unknown (including Magnemite, Voltorb, Baltoy, etc.), but they are all capable of breeding with Ditto.
Prior to Generation VIII, Bergmite and Avalugg were not part of this egg group.