From Merriam-Webster: Rust (v) : synonyms: corrode, oxidize, tarnish, decay, rot. I believe your confusion comes from failing to appreciate the degree of ambiguity in common English, rust being no different from many other terms. If one uses the definition that rust relates only to iron, then your original statement that "Gold and silver don't rust" is not only tautological, but banal. In general usage, a metal "rusts" when it oxidizes. Both gold and silver form oxides, and silver does so very quickly.