Yes, it is scratched off metal (you see it also on bathroom tiles if you use a bathroom wiper with aluminum frame for cleaning the shower tiles) or from using c

Yes, it is scratched off metal (you see it also on bathroom tiles if you use a bathroom wiper with aluminum frame for cleaning the shower tiles) or from using cast aluminum pans with machined bottom on glass ceramic cooktops (you get little aluminum deposits on the glass). It is reversible and you can "wipe" it off using acid and abrasives but it's tedious as the metal adheres really good to the ceramic.