Pajamas were cool in the early ’70s—and not just for nighttime. Spring collections showed evening dress that was a cross between daywear and bed­clothes. But how do you make loose knits and kimono shapes suitable for black tie? Add Van Cleef & Arpels. Take this navy ensemble ­modeled by Beverly Johnson in Vogue in February 1974: The jeweler’s diamond Rose de Noël clip secures her jersey cardigan over her slip dress.Van Cleef & Arpels created the Rose de Noël in 1968 after the civil unrest that May in France. “Before that, the elegant upper-class French woman was wearing jewelry for lunch, different jewelry for cocktails, different ­jewelry for dinner, and different jewelry for evening events,” explains Nicholas Luchsinger, Van Cleef’s vice president of North American sales. “Then came May 1968 and nothing was as formal as it used to be. Fashion was relaxed. Flo