purple Stain?

You know, I’ve never seen purple firescale, but I suppose it’s possible. Assuming there’s no stones or other foreign objects (i.e. it’s just straight silver), I’d dunk it in some pickle for a while. Your local jeweler should have some, and even if it’s not firescale, it should remove whatever it is. (Although, I must confess, I’m quite curious to know how you know about firescale without knowing about pickle.)