A ring turns a finger green because of a chemical reaction involving the metal mixed with acid naturally present in a person's skin. The higher the acidity level in an individual's skin, the quicker the discoloration occurs. The type and grade of metal of the ring also contributes to the possibility and speed at which discoloration occurs.
Increased heat, humidity and sweating also increase the rate at which discoloration occurs. A copper ring is a prime example of a metal that potentially causes a ring finger to turn green. The purer a gold in a ring, the less likely skin discoloration occurs.