Coal originated from the accumulation of plant fossils, which has taken placed for a long period of time, by million years, from both chemical and physical changes under proper temperature and pressure until those pant fossils are compressed and arranged themselves into the form of multi-layers and finally transformed themselves to become coal.
 

Coal from various sources will bear different physical and chemical characteristics, depending on period of time taken for accumulation, and environments of the source, from where they originated. When considering the coal characteristics based on quantities of humidity, ash, volatile matters and carbon content in coal, the types of coal can be categorized, according to the ranks of ASTM Standard, into 5 types, namely

ANTRACITE,
BITUMINOUS,
SUB BITUMINOUS,
LIGNITE and
PEAT.