I want to talk about staining as we seem to come across this a lot and there is great concern and confusion about what causes it and what to do about it. Staining happens to a piece of jewelry when the piercing hole is weeping and possibly on the verge of an infection. The acid in the fluid that come from the piercing stain the jewelry and cause a heavy oxidization.
This isn't something that happens because of the jewelry, its because the piercing is irritated and needs care. When you put a new hoop in it should be cleaned with rubbing alcohol. Clean the jewelry and the ear and wash your hands before handling the ring and putting it in your piercing. You can have something on your hands that will cause an infection if you are putting in a new ring or you could have something on your skin that will cause one so making sure you clean everything prior to changing out your jewelry is really important. Another thing that will cause this is if you have too thick of a gauge in your piercing. If the gauge is too thick the piercing hole can't get air and will become irritate. You are stretching your piercing when you use a bigger gauge and while stretching the piercing it requires more care.
Switching jewelry can irritate a piercing. I get a slight heat feeling and some itching when I change out my piercings and to combat that I make sure I clean everything before changing it out and I use an antibiotic ointment like Neosporin for a couple nights on the piercing to make sure the new jewelry doesn't irritate my piercing.
If staining happens you need to use an antibiotic ointment on the piercing with the ring in or out for a several nights and be sure that you are keeping the piercing clean. For the jewelry it needs to be cleaned. When you have a stain like this it is going to take some effort to get it off the ring but it will come off. Try a soft cloth and some toothpaste and rub the ring until the staining comes off. You can also buy a commercial jewelry cleaner made from whatever metal you have and follow the directions on the cleaner for cleaning. Steel wool will also pull the stain off the jewelry and polish it right up!
Some think if they develop irritation when changing out jewelry they have an allergy. This is not the case and you can easily tell if you have an allergy to the metal you are wearing. If it is an allergy you will see a very strange 'peeling' of the skin away from the jewelry. I hear about metal allergies often but have seen very few actual allergies to the metals!