Coal is very different from mineral rocks, which are made of inorganic material. Coal is made of fragile plant matter, and undergoes many changes before it becomes the familiar black and shiny substance burned as fuel.
Coal goes through different phases of carbonization over millions of years, and can be found at all stages of development in different parts of the world.
Coal is ranked according to how much it has changed over time. Hilt's Law states that the deeper the coal seam, the higher its rank. At deeper depths, the material encounters greater temperatures and pressure, and more plant debris is transformed into carbon.