On Oct. 2, 1960, just after dawn in Manhattan, Audrey Hepburn stood outside Tiffany & Co.’s Fifth Avenue flagship, Danish and coffee in hand, to film the opening of Breakfast at ­Tiffany’s. It was the first time the jeweler had opened its doors to a Hollywood movie crew—40 security guards and anxious employees were on hand to protect the stock—but Tiffany’s was getting something fabulous in return: publicity. That afternoon, Hepburn, still in costume as Holly Golightly, was ushered inside to pose for the press with the famous Tiffany Diamond in its new setting by French designer Jean Schlumberger.

Tiffany’s chief gemologist, George Frederic Kunz, studied the rough canary yellow stone for a year before slicing it into a 128.54 ct. brilliant shape with 82 facets. ­Schlumberger eventually set it inside a clip made of twisted strips of diamonds, platinum, and gold, and attached that piece to its matching Ribbon Rosette necklace.

The necklace, however, wouldn’t make it onto Hepburn’s neck in the film. Instead, it can be glimpsed in a display case when Holly takes her love interest to Tiffany’s. “It isn’t that I give a hoot about jewelry—except diamonds, of course,” she says, pointing at the gem. “Like that!”