Um... being in contact with skin would only increase the rate of tarnishing.
Yes if it's out in the open air oxygen will get to the copper and it will tarnish slowly, when worn, the sulphur from sweat and bodily oils will make it tarnish more quickly - as sulphur tarnishes silver as well as copper. Whats more it's a considerably harder tarnish to remove. Yes, if there's sufficient movement of a piece, say playing with a ring, or a bracelet that moves against a wrist, then this will be reduced or even eliminated by wearing the tarnish away as quickly as it forms, however there will always be a part of a piece which isn't so abraded (such as the inside of chain links) and it will show this accelerated tarnishing. The back of chains tend to be the most guilty party, and the backs of earrings can get pretty bad too.
Equally there very well could be a difference in our Sterlings as there's still a whole 7.5% which can be at great variance, it's usually just copper, but how pure is that copper? If there's already a high concentration of copper oxide in the lattice then tarnishing will be greatly accelerated. Equally insufficient pickling after soldering leaves excess copper oxide in the surface of the piece, if this isn't properly cleaned off then the silver will again tarnish more rapidly, further to this, if a piece is over-exposed to an oxidizing flame then the oxygen can penetrate deep into the silver requiring a long and arduous process to completely remove it, this isn't often done, resulting in a piece that will never look as bright as a perfect piece and it will tarnish even, faster. Whats more it's often plated over, resulting in a piece that looks perfect until the plate (usually pure silver so faster wearing) wears out, exposing the marred material beneath.
There is in point of fact a completely non-tarnishing silver alloy which contains 1% geranium - technically there is tarnish, but it's a thin coating of geranium-oxide, this doesn't affect the appearance of the item in any way and is the barrier between the copper and the oxygen which prevents tarnishing. Anything made from this is re-heated after polishing to ensure that the geranium oxide can form before copper oxide/sulphate.
Equally how was the piece finished? If it's simply soldered, pickled, and colored then it will oxidize much more quickly than if it is burnished before coloring. Soldering silver also anneals it, this loosens it's molecular structure adding to it's porosity, it also causes rapid and deep oxidation of the copper which needs to be cleaned by chemical etching (that pickle I've been talking about) polishing cloths and dips just aren't enough. This dissolves the copper oxide making the surface even more porous, if this is then colored then all those molecular gaps are still there - just waiting for oxygen, sulphur, whatever to join the party. If a piece is burnished (rubbed with a piece of polished steel/agate) then this smooths the surface in a way that simply coloring doesn't. The pressure causes the silver (and copper) to flow into the gaps, and toughens up the lattice reducing the rate of oxidation (the gaps just aren't there) and increasing the hardness to boot.
There's a really interesting process called reticulation, where you repeatedly heat Stirling (or a lower Ag alloy works even better) and pickle out the copper oxide, four, five, six times, however many it takes - but never too hot, just holding it at a low red. This leaves you with a surface which is copper poor above a substrate which is copper/copper-oxide rich. If you heat it very hot, very quickly the pure silver on the surface flows beneath the torch and crinkles beautifully.
Is it 925/1000 or a lower silver alloy? 800/1000 is still an acceptable alloy in the UK (hallmark-able no less) and US (and elsewhere?) it'll tarnish more rapidly due to the increased copper content but it looks exactly the same as Sterling silver. If someone were unscrupulous they could considerably lower that fineness and still maintain the silvery appearance. Was it really Stirling silver at all? I hate to say it but there is a hell of a lot of stuff out there that's simply stamped "925" as if owing a $20 punch makes it so. I've had someone bring me a piece for repair made from what appears (to me) to be aluminium that's been stamped that way. There's stuff calling its self "Peruvian Silver" which is anything but, samples tested have contained all sorts of rubbish, including one with a dangerous quantity of arsenic! That's why every piece I make will bare a hallmark, it's the oldest form of consumer protection in the world.
Again, yes, Sterling silver will tarnish, as I said in my second post. If you want to keep it immaculately shiny then it'll probably take a polish once a month or so, if you want to stop it from going "brown", once or twice a year should more than suffice. Unless you want a natural patina, in which case let it grow, and if you want instant patina, or a blackened cap (the top won't stay black) I can do that too. Frankly, I'd be more concerned by a buildup of copper sulphate on the tops of oft-unused keys, as I say, it's harder to clean, takes a bit more elbow grease.
If you want fine silver caps I can do that too, it doesn't oxidize even under an oxidizing torch (no copper see) However, it's considerably softer so much so that you'd have to be very careful with the stems when installing the caps.
If it's impossible to mark the inside of the piece, then it's impossible to mark the inside of the piece and the mark will be on the outside. The next time I'm in contact with the assay office I'll ask, but every piece will be marked.
