Chemical changes are accompanied by physical changes, such as decrease in average pore size. The macerals (organic particles) of lignite are composed of "huminite", which is earthy in appearance. As the coal matures to sub-bituminous coal, huminite begins to be replaced by vitreous (shiny) "vitrinite". Maturation of bituminous coal is characterized by "bitumenization", in which part of the coal is converted to bitumen, a hydrocarbon-rich gel. Maturation to anthracite is characterized by "debitumenization" (from demethanation) and the increasing tendency of the anthracite to break with a conchoidal fracture, similar to the way thick glass breaks.