It sounds like they're plated. Sterling is a homogeneous alloy, so you wouldn't see a layer of copper. If you send me detailed images of them, I can give you an estimate for replating.
My Web site is http://www.silversmithing.com/silver
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I really have no way of telling. I thought that since there is copper in sterling silver that it wouldn't be that surprising to see it. It is only visible if you look closely, best with a magnifying glass. I only noticed it because I felt the roughness when I rubbed my finger over it. I took them to a jewelry store and they said I would need a silversmith that does restorations to work on them. Now I just need to find one. I've had these earrings for almost ten years. I would really like to save them. If it is silver plate, what can I do????