Pure silver is often too soft to use in rings and can easily scratch and become damaged, so it is alloyed with copper to make Sterling Silver (92.5% silver + 7.5% copper). This is the meaning of the mark 925 for items made from sterling silver.
The "green finger effect" is a reaction to the copper alloy in the sterling silver, especially if you have high body acidity. It happens in about 2~3% of the population and unfortunately the percentage is increasing due to the chemicals in our environment and the food we eat.
Most jewelry is industrial plated with a fine layer of rhodium (rhodium is a form of platinum), which prevents this reaction from happening to anyone with the allergy. Not all silver rings are plated however, especially the antiqued look and oxidized rings since it will change the look.
Sometimes these reactions are occasional and happen only when someone eats something that has high acidity like tomatoes, is taking certain medications or even due to excitement.
- Soaking the ring for a day or two in a 50/50 mixture of household ammonia and water will usually take care of the problem.
- Another solution is to simply clean the ring every day with a silver polishing cloth. After a week or two, the reaction will normally stop occurring