Aeternalis Fuel is the highest tier of fuel introduced in Equivalent Exchange. It can be obtained through Philosopher's Stones or Energy Collectors and is necessary to produce Dark Matter and Red Matter.
Due to having the same EMC as Diamond, the two can be converted back and forth with no loss. This allows for more flexible crafting. As one is fuel and the other matter, they can't be converted in a transmutation tablet without a klein star.
Using a crafting bench and Philosopher's Stone, a full stack of Coal (64) is required to make one Aeternalis Fuel.
The name "Aeternalis Fuel" means "Everlasting Fuel" as "aeternalis" comes from the Latin word "aeternum" meaning "forever". This however is not completely accurate as aeternalis fuel doesn't actually last forever, although it is the best fuel in the game/mod.