The Crescent Sea
Heliite is a magical metal with adjustable density. By default it is lighter than air, and it was used in the construction of the world’s three flying cities. The skypirates of Freehold have achieved a higher degree of mastery over it, and understand how to near-instantly adjust its density from lighter-than-air to approximately that of steel.
Heliite is naturally occurring in very small quantities. The mage Cassius Marcus learned of a way to fabricate it from stone using magic. He has not taught anyone else to do so.