Petroleum

A technology lost to Avani civilization, Petroleum was a refined variant of Seafire invented by the Cyanni in the year 19 bt. Though in its natural form it is not very useful, in its refined form it could act as an extremely effective form of fuel. However, due to a lack of supply at the time it was never seriously pursued, and now its powerful secrets are all but forgotten in favor of its easily-weaponized cousin.

Utility

Historians have long theorized on how the world may have changed if this technology had been developed more, given more funding, used by a more stable nation, or any number of other things that could've seen its potential unlocked. Based on what records survived the Cyanni War, it was a highly flammable material that burned at high (but predictable) temperatures. Unlike other sources of fuel, such as coal or wood, it also provided significantly more energy for its weight and was safer to obtain (Coal miners usually work under extremely dangerous conditions).
Though this is all speculation, some believe all of these properties together probably would've made it the single most important technology in the world. It could've revolutionized land transportation, propelled advanced warships made of metal, fuelled terrifyingly massive factories, and unlocked resources unfathomable by current standards. Sadly, what it could've done for Avani civilization may never be known.

Access & Availability

None. Due to the political and economic climate within the country at the time, and a lack of access to the necessary resources, it was never advanced beyond a few experiments and promising prototypes. Over time the technology required to refine and store petroleum was lost entirely. Even with more modern technology and economic power its secrets have yet to be rediscovered by scientists.