The New York–born, Paris-based jeweler Joel A. Rosenthal, whose small atelier is known by his initials JAR, is recognized by a relatively small number of international jewelry lovers with the right connections to get an appointment at his Place Vendôme salon to buy one of his lavish creations. Despite his elusiveness, the press-averse designer is being celebrated with a large retrospective, Jewels by JAR, at New York’s Metropolitan Museum of Art through March 9, 2014—an unprecedented honor for a living contemp…