This is a vintage 12kt Black Hills Gold festoon-style (I believe it's called) necklace and is marked 12kt on the back of one of the leaves. It is about 18" long and the design is about 3" across. This is a vintage piece. Its chain does not react to a magnet--only the small clasp does. What is it worth and would you suggest I clean it before selling it? Is there a way to find out whether it's a Langstrom or whatever that brand is?