I got this $200 1992 gold coin and it has an unusual mark around the edge, that looks like some sort of tarnish? Excuse the stray fibre: Anyone got any clue what it is? I have no reason to believe the coin is a fake (if such things exist?)
It looks very much like the mark left behind by the connector clip used when electropolishing, to me. Does it appear on the other side, too? I used to electropolish stainless steel components used in building mass spectrometer ion sources, and it often left marks like that. I can't imagine why you would electropolish a gold coin though :/
yep, just a copper spot which is completely normal. Perfect world the alloy completely blended with the gold, but when you get microscopic flecks that aren't (and happen to be on the surface of the coin) they oxidise over time. Some people avoid coins with copper spots and other numi collectors look at them as a sign of authenticity. Infact copper spots don't have any effect on NGC grading provided they aren't eroded to the point of damaging the surface.