Sky-stones are stones that have broken off from the lower crust of the Astral Plane, usually during its incredibly powerful earthquakes.
The stones generally plummet quite a ways before the buoyency they experience in the Material Plane cuases them to float back up—usually not before smashing into the ground first though. Low density (density means the opposite here—it's the density of the "floaty" substance per weight of stone) stones float lower to the ground, while higher density ones can reach as high as the stars themselves (and are often populated by Xaphriel).
The lift of the sky trade
A sky ship does not float. It hangs. The hull is an ordinary sailing vessel slung on chains beneath a single sky-stone, and that stone carries the whole weight off the ground. Sails and Aether engines only move the ship sideways or trim its altitude; the buoyancy described above, chained and harnessed, is the one thing holding it in the air. A sky fleet rests, quite literally, on iridescent rock.
That rock is the most expensive thing in the trade. A stone large enough to lift a loaded hull costs more than a working captain can raise across a lifetime of voyages, so captains seldom own the thing their ship depends on. The stones belong instead to merchant houses and state navies, the bodies that already hold capital on that scale, and a captain sails beneath a stone he does not own. A master with his own lift has either inherited it or come by it in a way he would rather not explain over a drink.
This is where the sky trade's real lever sits. Lift pools where the money already is, so the routes answer not to whoever is bold enough to crew a ship but to whoever the stone-owners permit to fly. The houses and navies that hold the stones hold the trade itself, and a captain's ambition stops at the price of the one rock he cannot buy.
Sky-stone is only half of what keeps a vessel aloft. The other half is fuel: the Aether the engines burn and the Lanthornium that fires it, which drive a ship rather than lift it. These are separate purchases and separate chokeholds. One is the buoyancy a hull hangs from; the other is the propulsion that pushes it across the currents. A sky power needs command of both, and the trade belongs to whoever can pay for both at once.