The green to black discolorations on the skin underneath rings may be caused by either by a chemical reaction or allergy. Although cheap ring components like copper or nickel are notorious for causing such discolorations, gold and silver rings can also leave dark stains on the skin underneath the rings.
There are a variety of chemicals that can cause rings to leave a dark stain on the skin. One of the most common culprits is chlorine, as it has the ability to damage even gold rings. Ingredients in soaps, creams and lotions can also cause a chemical reaction with the rings. Dark ring stains can also be caused by a reaction of the metal with natural acids on the skin.