Conduit Crystals

Conduit Crystals are a rare mineral found deep beneath the surface of Theia. These purple gemstones have the unique property of exerting strong magnetic force when powered with enough energy.
Smaller Crystals require an oppositely charged pole and often lose their structural integrity when continuously powered.
Larger specimen are capable of interacting with the planets innate magnetic field and with enough power can lift entire cities into the sky. 
Conduit Crystals hold aloft the flying cities of Zephyros and Al'Karash (picture below) and more recently have been used in the creation of Skyships.