If you rub metal on some ceramics like a plate, porcelain sink, toilet, etc, it will leave behind a black mark. The mark can be removed by scrubbing. What is that black mark? Is it metal shavings, a chemical reaction with the ceramic, or something else?
It’s metal rubbing off.
You have re-created the touchstone.
Percelain has a very fine grain .
Ceramics used in the kitchen are covered in glass, glass has no grain, so as to avoid this effect.
And, being a liquid also makes it a lot harder to leave marks.
ceramic knife sharpening hones are full of those black marks. it is material removed from the metal object.