What intricate crystalline structure is revealed when an iron meteorite is etched with a mild acid?

Widmanstätten patterns

A key characteristic used to definitively identify iron meteorites involves examining their internal structure after preparation. When a specimen of an iron meteorite is cut and then etched using a mild acid, a distinctive, natural crystalline pattern emerges. These intricate geometric patterns are known as Widmanstätten patterns. Their formation requires an exceptionally slow cooling process, which only occurs deep within the cores of large parent bodies (planetesimals) over millions of years. Because this slow cooling cannot be replicated naturally on Earth, the presence of these patterns serves as incontrovertible proof of the object's extraterrestrial origin and its history as part of a differentiated planetary core.