Coudl this be The Dark Shore by A.A. Attanasio? The magical system is based on 'charm' which people use to (amongst other things) make floating cities
At the time of the Conquest, a loaf of bread in most dominions cost a newt's-eye. Small as a child's pinky nail and lustrous black with vermilion and emerald razor lines, the hex-gem known as newt's-eye carried the smallest possible charge of Charm—just enough to keep a healthy person awake one night, to heal a small wound, to boil three cups of blue tea, thread radiance through seven nights, stir a wisp of breeze for several hours, or buy an excellent loaf of nut bread still steaming on the baker's palette.
Dogbrick rubbed his head where the curb had kissed him and blinked,
perplexed, finding little to remember of Arwar Odawl. A floating city
far to the south. He knew of it only because of its famous brandy of
the same name—and also, of course, because it was renowned as the
oldest city on Irth.