Ceruleum
Ceruleum is a glowing blue energy source discovered and used by the Garlean Empire. Initially, it was used as a fuel to burn to keep fireplaces lit, and provide heat in the intensely frigid climate of Ilsabard. However, following the discoveries made by Solus Galvus, ceruleum was found to be a potent fuel that could greatly enhance the power of their magitek, from weapons, to transport, to the various devices used for everyday life.
It is no overestimation to say that the discovery of the efficacy of ceruleum as a fuel source changed the course of the Garlean Empire forever, allowing them to begin their swift conquest of two of the Three Great Continents.
Ceruleum is abundant on the continent of Ilsabard, but only shows up in remote portions of Eorzea. It has been farmed out of Ilsabard and only remains in a few places.
As ceruleum is incredibly volatile, it must be handled with extreme care. Any facility that houses ceruleum is heavily guarded to prevent entry by any party that seeks to misuse it, or any voidsent like Bombs and Grenades that are attracted to it. The effect of ceruleum igniting and exploding could be witnessed in Mor Dhona over Silvertear Lake, wherein the ceruleum tanks reacted to the damage caused by Midgardsormr and erupted, turning the entirety of the lake to crystal.
Lore
“For centuries, the Garleans warred with the Corvosi for control of the warm and fertile lands south of Ilsabard's central mountains, but they were eventually defeated and driven into the northern wastes. Winters in the Garleans' new home were harsh. The northern seas were bound in ice for most of the year, and the resources of the waters were available for only a slim window. Likewise, farming and rearing animals were possible only at certain times. The land, in other words, could not support a large populace. The Garleans multiplied slowly, and over time were forced to accept being few in number.
There was but one benefit to the frigid land—its vast deposits of ceruleum. This deep-blue liquid burned as well as oil, and kept the hearth warm during the long, dark times of year. Indeed, had it not been for this natural resource, the Garleans would not have survived the severe winter cold. Yet, it was to be a long time before the advent of the ceruleum engine, and the Garleans realized what a boon this liquid was.