Iron meteorites and the etching process that reveals their history

Iron meteorites are made almost entirely of a nickel iron alloy and are strongly magnetic, almost always the easiest meteorite type to identify with a simple magnet test. Their most striking feature only becomes visible after preparation, cutting a face, polishing it, and then etching it briefly with dilute acid reveals the Widmanstatten pattern, an interlocking geometric structure of two different nickel iron minerals that only forms when the metal cools extraordinarily slowly, on the order of millions of years, deep inside a large parent asteroid body. This pattern is a genuine physical record of that slow cooling history and cannot be reproduced by any terrestrial process, which is part of why it is considered strong confirming evidence once other tests point toward a meteorite.