. Fine silver is 99.9 percent pure or more, but it is very soft and unsuitable for most uses, Katherina. Sterling silver is defined as 92.5 percent pure or higher. Sometimes, as you note, 95 percent pure silver is sold; this has more silver than the minimum it needs to be called sterling but it's still sterling.
The remaining 5 to 7-1/2 percent is usually copper because this has been found to be the best metal to alloy with the silver to not discolor it, while making it hard.
It is generally considered that the cause of silver tarnishing is sulphides in the air rather than oxygen. Maybe you lived in an non-industrialized village in Peru and you now live where there are a lot of diesel trucks and other sources of sulphides?
The jewelry manufacturers try various preservatives to keep the silver from tarnishing, including rhodium plating, lacquers, and corrosion inhibitor treatments; as far as I know, there is no 'standard' saying that the jeweler can or can't use these techniques, so it is difficult to know which is any were used on the jewelry you are buying. Good luck.
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