I wasn't doubting that the coins could be scratched, just whether the resulting microscopic flakes could be harmful in any meaningful(perhaps significant is a better word here) way.
A good example is that I recall reading a post in a thread somewhere that if you vape at a high enough temp, you can start producing benzene(I think it was benzene, but it doesn't really matter for this example
).
Benzene is bad, yes, and it should be avoided, yes, but another poster pointed out that the amount of benzene you'd get at very-high-but-not-combustion temps is equivalent to what you'd get when you smell petrol when filling up your car I.E. a negligible amount.
A real concern based in fact, but not something I'd personally worry about.
I was actually gonna see if I could look up the hardness figures for my grinder and coin, but then I... Couldn't be arsed doin it