There are a few reasons why a gold ring sometimes leaves a dark stain on the finger, but most of these reasons involve chemical reactions. Chemicals, alloys and acid both from the jewelry and the wearer, can react in different ways to leave a dark stain, which is often green or black on the fingers.
Although gold is resistant to damage and degradation against most chemicals, chlorine has the ability to eat away at it. Chlorine also has the same eroding effect on sterling silver.
In some cases, the ingredients in lotion or makeup can also eat away at the metal alloys on the ring. This is because some of the ingredients in lotion or makeup are harder than the metal, which also causes an eroding effect on the metal alloy on the ring. As the ring erodes, microscopic particles are dislodged from it and attaches itself onto the skin on the finger. Since these microscopic particles do not reflect light, they leave a dark stain, which ranges from green to black, on the finger where the ring was worn.
There are several ways to prevent rings from leaving a dark stain on the finger. Removing a gold ring prior to taking a dip in a swimming pool, will keep chlorine from coming into contact with the ring. It will also be a good idea to remove rings prior to handling any household chemicals to prevent any chemical reactions. Having the ring plated with rhodium also helps in preventing rings from staining the fingers.