“Their scantly leav’d, and finely tapering stems,” wrote John Keats in a poem of 1816, “Had not yet lost those starry diadems / Caught from the early sobbing of the morn.” The starry diadems—parures and tiaras, too—now on display in Monaco are not dewdrops but diamonds cut and set by the Paris jeweler Chaumet, whose first great client was Empress Joséphine. Created for royal, aristocratic, and important European women of the 18th century to today, these more than 250 treasures, symbolic and exquisite, come from 15 museums and 40 private lenders. —L.J.