Lanthornium is an uncommon gray-blue stone, and like the Aether it answers, it does not belong to Alaria. The two arrived together. An eruption on the neighbouring stack of Lanthornia threw a great quantity of Aether and Lanthornium clear of that world, and both came to rest here, scattered through a world whose own laws never made either. It is no native mineral, and it is no leaving of the lost aetherial height — that height was Kethic work raised by mortals and lost across the Lost Ages, and it shares nothing with the foreign Aether but an unlucky echo in the name.
Raw Lanthornium barely does anything. What comes out of the ground is dull and unresponsive, and bringing it to the state an engine needs is the expensive half of the whole trade. Purification reduces it to a worked gray-blue plate, and the cost runs steep enough that the sky-trade prices the refined plate rather than the rock it came from.
The struck plate
A purified plate is set vibrating at one exact frequency, and the engine is built around that single trigger. Processed Aether brought against the vibrating stone strikes it and changes all at once. It expands, sheds the bond with the water that held it as fuel, and discharges as it slips back toward the insubstantial it came from. That discharge is the work — the shove that drives a piston, the turn that spins a propeller. The plate is not spent in the striking; the Aether is.
A good plate does not need striking again by hand. The first discharge sets up the vibration that triggers the next strike, and that one the next, so the plate carries its own rhythm the way a steam engine carries its own fire. From there the engine runs itself, a sustained cycle of strike and discharge that holds for as long as fuel and stone last. What it releases, though, is an off-world substance unspooling back toward nothing, not heat off anything burning. The principle copies into no foundry and grows to no grid. It moves sky-ships and a thin handful of machines, and there it stops.
The price of an engine
The stone is only where the cost begins. An Aether engine wraps the plate in a mechanism that has to hold it at frequency and feed Aether against it in measured strikes, and building and tuning that mechanism takes knowledge held by few hands, so the finished engine stands among the largest single costs in a sky-ship, laid atop the fortune already sunk into an astral stone and its astral-steel chains. A plate that cracks or a mechanism knocked out of true can ground a vessel until both are set right. That is why an engine travels with the people who understand it, and why the understanding is guarded as closely as the fuel.