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If you have a metal clasp where the finish has rubbed off, recolor it with a metallic pen then seal it with clear nail polish or other sealant.
You can usually also replace the clasp. Think about using a clasp that will not have the finish (plating) wear off. The investment in a Sterling clasp, for example, is not really very much for a piece of jewelry that you really like. In most cases, you can cut the old clasp off without disturbing the piece - then attach the new clasp using split rings.