Ceruleum is an energy source in Final Fantasy XIV. Ceruleum is a refined form of Aether, obtained by draining elemental Crystals of their energy, usually in processing factories. It is a powerful yet highly unstable source of energy, mainly used by Garlean Empire to power Magitek engines.
The Garlean Empire consumes vast amounts of Ceruleum to fuel their Magitek engines, powering anything from airships to armored tanks. In contrast, Eorzean citizens mostly use Ceruleum to just power their airships.
Though it is abundant on continent of Ilsabard, in Eorzea, Ceruleum is only found in a small swath of mountainous land located in northern Thanalan. These Ceruleum deposits are monitored and controlled by the Amajina & Sons Mineral Concern of Ul'dah, as it fell within the city-state's land and they recognized the potential value of ceruleum. Ironically, before Gaius van Baelsar invaded Eorzea, the imperial army was the Concern's biggest customer. And now, the XIVth Imperial Legion installed a competing mining facility near Castrum Meridianum. Due to the bluish haze produced by evaporated Creuleum, the entire region became known as "Bluefog" due to the perpetually tinted sky.
Ceruleum is very volatile and must be handled with care. Worse, it is known to attract voidsent grenades that can easily ignite it. For this reason, security is often high near any facility that processes the fuel. During the Battle of Silvertear Skies, as the Imperial flagship Agrius fell in confrontation with the Dravanian Horde, its ceruleum tanks reacted with aether released from the dying Midgardsormr, which resulted in a dramatic crystallization of Mor Dhona's landscape.