They possess, inevitably and unavoidably, an element of fancy dress. They are the very heart and soul of dressing 'up'. They are fairy tales in diamond form; ambition achieved and glory well worn. They are head-medals, really. Of a kind. But so tremendously beautiful. So twinklingly glamorous. They put stars in your eyes and roses in your cheeks and it's hard not to feel lucky when you're wearing one because you haven't just struck gold, you've struck tiara.