This struck me as very surprising. Straight Dope's take[1]:
> While coal-burning power plants are responsible for producing most of the sulfur dioxide out there (and thus acid rain), they don’t contribute that much of the compounds that actually cause silver to tarnish, namely hydrogen sulfide—best known as a key player in the smell of rotten eggs and flatulence—and the similarly pungent carbonyl sulfide. About 90 percent of the hydrogen sulfide and more than two-thirds of the carbonyl sulfide in our atmosphere come from (you guessed it) volcanoes, salt marshland, undersea vents, and other natural sources.
Before the industrial revolution, silver did not tarnish. Burning coal introduced enough sulfur in the air that silver started tarnishing.