The Black Key by Amy Ewing

For too long, Violet and the people of the outer circles of the Lone City have lived in service to the royalty of the Jewel. But now the secret society known as the Black Key is preparing to seize power.

And while Violet knows she is at the center of this rebellion, she has a more personal stake in it—her sister, Hazel, has been taken by the Duchess of the Lake. Now, after fighting so hard to escape the Jewel, Violet must do everything in her power to return to save not only Hazel, but the future of the Lone City.