In many instances, high-end fashion jewelry has achieved a "collectible" status, and increases in value over time. Today, there is a substantial secondary market for vintage fashion jewelry. The main collecting market is for 'signed pieces', that is pieces which have the maker's mark, usually stamped on the reverse. Amongst the most sought after are Miriam Haskell, Coro, Butler and Wilson, Crown Trifari, and Sphinx. However, there is also demand for good quality 'unsigned' pieces, especially if they are of an unusual design.