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.
Flight-rocks began as almost imperceptible bumps on the ground in the Stone Gardens, but would grow over time, pushing other rocks upward and expanding. It could take a rock decades to reach maturity. The upward push from rocks below resulted in towering rock stacks in the Stone Gardens, with the larger rocks nearer the tops of the stacks.
Typically, large sets of flight-rocks matured at the same time. Ripe flight-rocks produced vibrations detectable by the White Ravens roosting on the top of the rock stacks. When rocks became large and buoyant enough to break free and soar off into Open Sky, they produced an eerie howling noise, and the white ravens would respond by forming screeching flocks. The combined howling from the rocks and screeching from the white ravens was known as the Chorus of the Dead, and most of the citizens of Undertown were terrified of it. The common Undertown belief was that the rocks howled because when Sanctaphrax academics were laid to rest in a traditional ceremony in the Stone Gardens, their spirits entered the rocks and possessed them.
The flocks of white ravens heralding a set of ripe stones would fly from the Stone Gardens to Sanctaphrax, circling the taller towers and screeching to alert the academics that it was time to harvest the flight-rocks. The academics fiercely guarded their position as the sole harvesters of flight-rocks, and exacted severe punishments on trespassers, sometimes even execution. What was more, those Undertowners who attempted to harvest their own flight-rocks usually failed, for the academics did not share their methods for successful flight-rock harvests.
Harvesting flight-rocks involve using white hot callipers to hold the rock so that the rock would heat up and sink down. Then the academics used nets to bring them to the ground. After, they took the rocks to a "rock ballif" who would inspect it and report it to the leaguemaster.
Because the academics were the only individuals capable of harvesting the flight-rocks successfully, the League of Free Merchants paid them handsomely for their services. Once the flight-rocks were harvested, the Leagues would load them into a cart and send them directly to the Sky Shipyards.