Crystim

Quartz dug from the Wall and surrounding petrified mountains are not only powerful objects by themselves, but when ground into a powder, mixed with saltwater and secret ingredients and enchanted they produce a fuel cell equal to an engine in our world. This material is called Crystim, and the larger the resulting crystal, the more power it provides. Very select few individuals are ever taught how to make Crystim, and only they know the secret ingredients, guarding the knowledge with their lives and their deaths. Many often try to pry the information via espionage to make their own ends with it, but the level of security the holders of the secrets have around them rivals an extremely high-level dungeon or adventure rating. Adventurers gaining their license are often schooled against trying for such jobs, as not only are the pursuits unbecoming and unheroic but also prohibited by law in almost every part of the World, where such folk prefers to reside for the additional security.
People who are inducted to learn the ways have many factors to their training. Ethics, so as not to deplete the ingredient origins to extinction; economics, so as not to flood the market or lessen the demand to novelty; logistics, so as to manage creation now vs availability in the future, lest they are tempted to match demand instead of maintaining viable market; Magic, so that they can actually make the items; and naturalist studies, so as to recognize legitimate ingredients, lest they mix in a fake and ruin all the other ingredients in the process. You will never find an uneducated, lazy, greedy Crystim Maker. Generally, they are described as difficult, cold, calculating, intimidating, and aloof for commonality descriptors. There are variants in as much abundance as individuals in the training. They are taught far more than they need to know to ensure the future generations do not diminish in learning only the basics. The thought of someone less than adequate taking up the mantle disgusts all who have it since that would lead to ecological disaster very swiftly should a weak undesirable be swayed by lesser purpose.
The Crystim itself is rarely wasted on anything banal, though some legitimate salvage is utilized for basic transportation in devices used by such as Publishing and Printing devices, Caravan transports, and even aqueducts throughout the Wall (though usually, the natural Quartz is strong enough to maintain those as the Wall power is close at hand to keep them charged). Most often, long-distance intercontinental airships of great renown, the trains that run through Tunnel Town, and ships of immense value are the sort to receive Crystim on such a minor level. Cities, floating islands, Spaceport & their ships, and other such vessels of magnitude that serve more of humanity than the few are the calling that receive such outstanding power sources.