Silver tarnishing with unknown cause

[Dennis]

That has been a regular topic over the years, with never any clear solution. One thing is certain: You will get the blame for apparently passing off inferior metal.
If none of your guesses solves it, then re-fund and move on. Dennis.

[CJ57]

On other forums silver immediately going black ‘without reason’ often means they have either been in the hot tub, swimming or usd other such cleaning agents, more than likely with rings. Could be hand cream, perfume, hairspray, who know really what someone does once they get something home

[mizgeorge]

You'll find that one of the biggest problems is that people expect the same from their handmade pieces as they get from the mass-produced stuff they're used to, which has almost inevitably been plated with fine silver rather than being properly polished, and as a result doesn't really start to tarnish until that fine outer layer has worn away (as it inevitably will).

If it's a higher value piece, I'd recommend including a small cleaning cloth (even one cut down from a larger one) and a little explanation that silver will tarnish when in contact with many different things, but that it can be brought back to a high shine with just a tiny bit of effort.