Oil

Oil is a yellowish black-liquid found underground that is oftentimes processed into a petroleum-derived flammable liquid, which serves primarily as fuel for spark-ignited combustion engines, such as many high-power mechanical engines found on Engineer and Engineer's Navy vehicles. Derived primarily from organic compounds, the petroleum is distilled and processed into the oil through refining processes. 'Oil' is also a bit of a catchall term for a variety of petroleum based products from a variety of sources, as there are many formulations and formulas for different types of oil, that can be vastly different than one another, such as lubricants and emulsifiers and even medical supplies, but in general they carry some type of petroleum-derived base.
Oil as a fuel is being further explored, but it has proven significantly more reliable than other sources of propulsion like coal, clockwork mechanisms, or wind. It is also something that is usable by all, as compared to something more magical in nature, which The Engineers would not have access to. Many of the larger and important machinery in the Engineers' society rely on oil. Alchemists are continually finding more uses for petroleum based products, not only for machinery, but medical and daily life applications as well. 
The main source for oil is through underground excavation, which occurs primarily on the micro-island of Ka'ti to the west of Badull. There is a large amount of oil beneath the Darkvale Forest, which the island's owner, Kai Li, exports to The Engineers.
There are some that postulate about its danger to the environment, but further studies are needed in regards to this.

Type

Biomaterial