Tarnishing is one of the major issues people have to deal with after owning a ring for a while. Tarnish is essentially a layer formed by the corrosion of a metal, such as Sterling Silver, for instance. The metal chemically reacts with compounds it comes into direct contact with such as oxygen or sulfur dioxide. This happens in part because of the way the jewelry is stored and used.
Cleaning and Polishing
Regular cleaning of diamonds prevents lotion and body oil form building up on the jewelry piece, especially if you do it each time you take it off. Alternatively, you can wash the item in soapy water and then thoroughly dry it. A good way to remember to do this is to clean your rings once every major Holiday of the year.
It is easier for jewelry to tarnish when it is left in the open. Keeping it inside an acid-free container that is kept closed most of the time can fix that problem. If you do happen to leave your jewelry in the open, make sure it is not under direct heat or sunlight. For good measure, wrap it using a soft cloth. This keep the elements off the surface, making the diamond studded ring less likely to tarnish.