One of the two mineral sources of fuel, Lignite is found in veins in sedimentary layers. When processed at a smelter or magma smelter, one unit of lignite produces 5 units of coke. If done at a regular smelter, this processing requires one pre-existing unit of fuel (either charcoal or coke), leaving a net production of 4 fuel.
Lignite is flammable - if exposed to fire or magma, an item made of lignite will burn for the better part of a year before wearing away. Exposure to water (including rain) will extinguish it, unless it happens to be stored in a bin.
 See also
[SOLID_DENSITY:1250] Should be less dense than bituminous coal.