The story
The Seattle Convention and Visitors Bureau held a contest in 1981 to find a new nickname for Seattle. The winning entry, "Emerald City", came from Sarah Sterling-Franklin of Carmel, California, whose submission described the city as "the jewel of the Northwest, the Queen of the Evergreen State, the many-faced city of space, elegance, magic and beauty." "Emerald City" is also a reference to the fictional capital of the Land of Oz in the book series by L. Frank Baum.
Etymology from Wiktionary, CC BY-SA 4.0.