As befits a city that's a couple hundred centuries old, Istanbul boasts its share of awe-inspiring landmarks and architectural marvels. Gilan, the 32-year-old family-owned jewelry firm based in the ancient Turkish metropolis, has crafted pieces—and even entire collections—in honor of the city's greatest: the Ottoman royal residence–turned–museum Topkapi Palace (jewels pay homage to its gardens with Gilan's trademark tulip-cut stones and rose-cut diamonds); the Bosphorus Bridge (ocean-blue chalcedony and waves of sapphire represent the river running between Europe and Asia); and—with this gold, rubellite, garnet, and diamond ring, part of Gilan's new Colors of Emotion ­collection—the Hagia Sophia. "It was built as a cathedral and then converted to a mosque," says international marketing manager Osman Gilan of the renowned basilica, which now functions as a museum. "Now it is