Traditional. Conservative. Demure. These words aren’t usually used to describe Madonna—or the slinky Bob Mackie gown she wore to the 1991 Oscars, where she performed the nominated song “Sooner or Later” from Dick Tracy. But her Harry Winston accessories were another story. The singer borrowed several million dollars’ worth of jewels, including the firm’s now-iconic diamond cluster necklace, for the event. Winston had conceived the design—which features bunches of mixed-cut diamonds set together in platinum—nearly 50 years earlier after studying the Christmas decorations outside his Scarsdale, N.Y., home.“Imagine taking a holiday wreath and putting it around your neck. But where the green needles would land, there are diamonds,” says Judith Price, president of the National Jewelry Institute. “The cluster necklace is beautiful, but if you compare it to the modern diam