Has this happened to you with any other sterling silver, or base metal (costume jewelry) rings?
Has the ring come into contact with sweat or any chemicals (like cleaning sprays)? Did you wash your hands or shower wearing it? Remember that silver is nowhere near as durable as gold alloys when exposed to elements (I've seen some horrible oxidation and pitting on sterling silver jewelry that's never been taken off).
Why your skin turn green?
It's definitely not because it's faulty, it could be your body's reaction to the silver. Sterling silver is 7.5 percent copper, when it comes in contact with your skin, a chemical reaction occurs that causes the green color. This reaction happens more commonly with sterling silver than with gold.
And why my sterling silver will turn black ?
Sterling silver jewelry become darkens due to a reaction with gases in the air.
Is there any way to avoid these problem ? How to do it ?
1. Clean the inside of the ring thoroughly (and not just with a cloth, and NOT with soap, toothpaste or anything that would leave buildup). Sometimes that helps.
2. Clear nail polish is what I would usually recommend to someone with a reaction but who still wants to wear their jewelry. However, it will wear off, especially if you wear the ring constantly and/or sweat.
You can coat the inside of the ring with clear nailpolish. And the ring will never turn your skin green again and this way also keep your sterling piece in shiny and well condition!