Siblings share clothes. Monarchs share tiaras. The headpiece on Britain’s Princess Margaret at this 1951 Parisian charity ball actually belonged to her older sister, Elizabeth II. Their father, George VI, had purchased the Cartier London jewel for his wife, Elizabeth Bowes-Lyon, three weeks before they became king and queen in 1936.“Not too many people were buying new tiaras in the 1930s,” says Margaret Young-Sánchez, coauthor of Cartier in the 20th Century (Vendome Press). “It was primarily an English phenomenon because they still had a royal court.”This one boasts 739 brilliant and 149 baguette-cut diamonds in platinum, forming 16 graduated swirls or scrolls. It is known as the Halo Tiara, says Young-Sánchez, because of its position on the head: “Rather than sitting horizontally like a circle, it stood vertically with the band coming down behind her ears.”The Queen Moth