Talk about the Cinderella treatment: book your wedding reception at the Mandarin Oriental, New York (80 Columbus Circle; 212/805-8800; and you can borrow pieces from the jeweler Lockes Diamantaires (10 Columbus Circle; 212/823-9511) at no extra charge. The extraordinary arrangement lets brides and their mothers play dress-up from a collection of traditional and custom-designed pieces, such as the pear diamond pendant, which sells for more than $183,000. Fickle princesses can choose new trinkets for different outfits. Alas, when the ball's over, the jewels go back.