Iron meteorites are made, almost completely, of iron and nickel metal. They are chemically distinguished and grouped according to the abundances of the trace elements such as gallium and germanium, as well as nickel. Initially, irons were classified into four groups and were given Roman numerals I, II, III, and IV. Today twelve groups are recognized and designated further by letters A through F according to concentrations of siderophile ("iron-loving") trace elements. Iron meteorites that do not fit into the groups are called ungrouped. The two iron-nickel alloys in iron meteorites are called kamacite (low-nickel content, usually up to 7. 5 wt% nickel) and taenite (high-nickel content, ~20 to 50 wt% nickel). These alloys are rare in terrestrial rocks. (See also Widmanstätten Pattern. )