Ceruleum is a powerful yet highly unstable source of energy in Final Fantasy XIV, mainly used by Garlean Empire to power magitek engines. The Empire consumes vast amounts of it to power 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 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. The XIVth Imperial Legion has installed a competing mining facility near Castrum Meridianum. As evaporated Ceruleum produces a blue haze, the region became known as "Bluefog" due to the perpetually tinted sky.
Ceruleum is volatile and must be handled with care. It attracts voidsent grenades that can easily ignite it. Security is 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 crystallized Mor Dhona's landscape.
Not long after Emperor Varis was murdered by his own son, Zenos yae Galvus, the Empire descended into civil war. The Imperial Legions tearing Garlemald apart in their efforts to achieve their own interests, the price for Ceruleum has shot up all across Eorzea.