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When silver rainbow tones, it is beautiful. Some collectors love this and will pay a premium for the beautiful natural colorful toning. However, black (terminal) toning is a different story. Usually not favorable. But, yes, toning is corrosion and tarnishing. There is still valuable silver underneath.

It tarnishes, and tarnished silver does not affect the value :) This is a poured piece of silver with a polish on it, I doubt it will tarnish, at least it won't for a very long time.