Oxidized silver has a tarnished look caused by the compound silver sulfide, which forms when the metal is exposed to air containing small amounts of sulfur. Silver sulfide darkens the finish, creating blackened patches. Some silver jewelry is treated to purposefully give it this appearance; other times it can be unsightly, particularly if the oxidation affects something like utensils.
The darkened appearance of oxidized silver gives jewelry a vintage look; if this is the goal, there are ways to speed along the necessary chemical reaction. To oxidize silver quickly, Beading Daily suggests placing it in a bag with two halves of a boiled egg. In one or two days, the sulfur in the egg will oxidize the metal naturally.
Silver sulfide can be removed easily, giving the metal back its shine. Abrasive polishes remove silver-sulfide buildup, restoring luster; some dissolve the substance, but also strip away some of the precious metal. Other polishes reverse the original chemical reaction, turning the silver sulfide back into normal, shiny silver.
References and ResourcesUniversity of Wisconsin: Put a Shine On It
BBC: Oxidation and Reduction
The Auglen Mineral Index: Why Has My Jewellery Changed Colour?
Beading Daily: The ABCs of Oxidizing Jewelry
ResourcesScotweb: Caring for Silver Jewellery
Live Science: Facts About Silver