Assuming it doesn’t come with the set there is a whole other design feature that you need choose when buying a window treatment. What style rod matches the decor of the shades and cornice. A special shade stopper could add plenty of elegance to otherwise plain window. Unless it is completely covered by the cornice the rod or stopper is always noticeable. Of course you need to find something that could go room design without being too large fancy.
A tip to follow for matching a style is, unless you are going for a specific historic style the simpler the better. It is also good to note that black goes with most colors. Crystal could end up being pretty transparent which could hide the stopper altogether.