Most iron meteorites contain very interesting patterns in their cross-section (after cleaning but also on the outside). These are so-called Widmanstätten patterns, the lattice-like arrangement which resembles a regular grid that appears on the cross-section of an iron meteorite when it is polished and etched with acid to highlight the patterns. They are related to the crystal structure of the meteorite, formed by plate-like crystals of kamacite and taenite. Both minerals are composed of iron and nickel, i.e. they are not found on almost pure iron meteorites (ataxites) such as Dronino.
These patterns were described in 1808 by Count A. J. von Widmanstätten and are clear evidence of extra-terrestrial origin and have never been observed in terrestrial iron. They are said to have been formed during the cooling of metal in a weightless state for several million years.
Muonionalusta, Gibeon and Seymchan, to name a few, are among the meteorites with the most distinctive Widmanstätten patterns. Interestingly, different meteorites also vary greatly in the thickness of the "lines" that make up the patterns. To make them visible, the surface of the meteorite section needs to be polished, and then etched with a mild solution of nitric acid; a solution of nitric acid in methanol (called nital) is optimal.