The pictures are the best description. When higher end jewelry isn't signed it does not, necessarily, decrease its value. Many of the vintage jewelry designers throughout the 1900's made jewelry for other jewelry houses. Juliana is a great example as you will not find any Juliana jewelry marked (other than a paper tag). Weiss, Kramer, Miriam Haskell, Schreiner are some other companies that didn't sign all of their pieces.
Generally it will be the earlier pieces that were unsigned.