In coal classification, which term refers to the hardness of the coal?

In coal classification, the term that refers to the hardness of the coal is "rank." Coal rank is determined by the degree of metamorphism that the coal has undergone over geological time, which affects its carbon content, energy density, and therefore its hardness. Higher-rank coals, such as anthracite, have the highest carbon content and hardness, making them denser and more energy-dense compared to lower-rank coals like lignite.

Other terms like "grade," "type," and "quality" have distinct meanings in coal classification. "Grade" generally refers to the classification based on use and economic value, while "type" may indicate the specific kind of coal based on its origin (like bituminous or sub-bituminous). "Quality" often encompasses multiple factors, including chemical composition and physical properties but does not specifically denote hardness. Thus, rank is the most precise term for indicating the hardness of coal.