The Mackay Emerald Necklace, designed by Cartier and set with a whopping 167.97 carat Colombian Emerald, is displayed at the Smithsonian Institute’s National Museum of Natural History in Washington, D.C.; the Art Deco necklace was made in 1931 as a wedding gift from Clarence Mackay to his second wife, Anna Case, a soprano who sang with the New York Metropolitan Opera