You have a lot of silver jewelry, but you've noticed that while some pieces require constant attention to keep them shiny and new, others never seem to tarnish.
Rhodium is a shiny silver metal with a finish almost like mercury.
This finish is good because it mimics the brilliant finish of freshly polished silver and protects the piece from natural tarnishing, but it also has its drawbacks, such as uneven wear, scratching and repair difficulties. It doesn't always wear off evenly, so you can get dull spots on the jewelry. There is nothing you can do about it, except to have the jewelry rhodium plated again. Rhodium does not stick to stones, so a piece already set with stones can be rhodium plated, but the process may be cost prohibitive - sometimes costing more than the jewelry is worth in the first place. But if you have a cherished piece you simply don't want to give up on, call around to your local jewelers and tell them you have silver jewelry you want re-plated with rhodium and see if they can provide that service.