You can test if it is sulfur based oxidation by doing this, (it works primarily on sterling but you can give it a try...........)
a ceramic bowl lined with aluminum foil, shiny side up
a tablespoon of baking soda
a tablespoon of salt and some hot water.
Put just enough water in the bowl to dissolve the baking soda and salt and cover the item you want to clean up.
stick the silver thing in it and see if the sulfur flakes off after a few minutes and moves to the aluminum foil. Dry whatever you stuck in there with a towel and see what you have. I know some jewelry folk put the soda and salt on the item and then pour the water on it.......my sister used to add tiny bit of dish soap on her sterling Native American Jewelry at the shop she had years ago, then buffed it up. She said it takes a bit of practice to get the consistency correct but it works after about 5-15 minutes of soaking as I recall.
updated by @salt-springs: 03/01/23 12:23:19AM