It is generally copper that discolors. Brass and bronze have a large amount of copper in their alloy also causing the discoloration. Unfortunately, rings are so prone to wear, there is nothing long term that will protect them from interacting with a person’s skin. I have tried clear nail polish, lacquer, protecta clear, etc. (You also need to be careful of harmful chemicals with any coating) Gold plating may be the best but it is still only temporary, until the gold wears off. The best thing to do, if you make rings containing copper, is to let the customer know about the discoloration and why. It is just a chemical reaction with the skin. Some people get it more than others. There is also some that believe the copper can help with arthritis. This can be a benefit. I have met one person who had a negative reaction to silver. That was a first. (Perhaps a vampire?….eek!)