Flight-Rocks were buoyant boulders harvested from the Stone Gardens and put to use in sky ships. They were tended by a Stone Pilot, and were the source of a sky ship's buoyancy. Cooling flight-rocks would cause them to rise; heating them would force them to sink. Flight-rocks were present in all sky ships, although some smaller sky barges used rubble instead of an intact flight-rock. Flight-rocks were actually living organisms, one of the three Ancient Ones.