The Petina finish on coin rings is applied for many aesthetic reasons, the main one being how the petina makes the design on the coin jump out.
Coin rings made from silver, copper and bronze can be given an Antique petina.
The Petina is created by artificially aging the surface of the ring by oxidizing the metal with a chemical called Liver of Sulphur.
Liver of Sulphur can create some very interesting if unpredictable colours from blues, greens through to black
The Petina is not permanent and can be easily removed with metal polish. Normal wear and tare will also eventually remove the petina. I do seal the petina which helps preserve the finish, the ring can always be retreated to restore the petina at any time, which I am quite happy to do for a nominal charge to cover the cost of the chemicals used. Alternatively you can always refinish your ring yourself, I will happily provide full instructions.