Have you ever purchased a new ring then wore it around and after a while it begins turning your skin a green hue. For many people this is a regular occurrence and many have no idea as to why this is happening. When your skin is turned green because of this it means that oxidation of either the skin or the ring is taking place. What happens is many gold or silver jewelry pieces are made of mostly copper, where the manufacturer will use copper as a way to be able to add weight to the ring while keeping the price low, since copper is less expensive than gold or silver. Then smaller elements of the more expensive material such as gold or silver are used to give the ring its shinny appearance.
When copper is exposed to your skin and the air it begins the process known as oxidation. This is when the microscopic elements begin to break down the elements of copper. As you are sweating this oxidation will then begin turning your skin and the elements of the ring green. This is because the chemicals of the body is interacting with the copper causing it to break down, making the ring that you are wearing and your skin slowly turn green.
For most this can happen in either a single day or even over a period of weeks, so if it happening to you, now you know why. There are not many solutions other than to get jewelry that is lower in copper content than any other metals present, although that approach can be considerably more costly.