Jean Schlumberger. Among the cognoscenti he’s considered the greatest of 20th-century jewelers. He mixed precious and semiprecious but was a purist, thought diamonds a bit vulgar and loved less common stones—peridot, moonstone. His genius was in his designs. And he could be inspired by clients, especially an imaginative and moneyed client like Rachel Lambert “Bunny” Mellon. In Qatar, in an exhibition that kicks off its new National Museum—a rapturous complex designed by Jean Nouvel—Schlumberger showstoppers from Mellon’s collection, on loan from the Virginia Museum of Fine Arts, is a glittering overture. —L.J.