Have you seen this? In fashion magazines they are showing necklaces that close with a piece of grosgrain ribbon tied in a bow. It looks kind of Chanel to me. Usually they use a 3/8 inch piece of black grosgrain ribbon, which is about 80 cents a yard at the fabric store.
If you have a necklace that is not quite long enough to wear doubled, you can double it & loop the ribbon through each end, tie it on your neck with the ribbon in a bow. Obviously some necklaces are too short to look good this way, but many would.
Give it a try!