I have several antique pins and brooches. The obvious ones are marked 925 but there are many that are not marked. None of them stick to magnets and all are heavy for their size. And, as far as I can see, none of the metal has been chipped away to expose a possible secondary metal underneath.
I do have one piece, however, that seems to be tarnished in a small area, but will not come off with my polishing cloth. It doesn