Heavy moisture/humidity will cause silver to tarnish, if they stay in that environment for long they can go completely black.
Here is one such silver ring.
Found in an area where I also found a silver quarter that was so tarnished and black I missed it and didn't notice what I had until after I tumbled it.
This ring was also found in the same site and area which was the bank of a long forgotten tiny fishing pond buried in the middle of the woods.
The moisture that never really went away from where these silver pieces laid worked on them for years so they both turned black.
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I didn't suspect it was silver until I saw the 925 mark, I never saw anything silver tarnish this bad.
I used a similar technique explained above, place the object in a piece of foil in the bottom of a pan with a heated up slurry of baking soda and water because there was no way I was able to get in there and clean it by hand polishing.
Trust me, this method works.
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