Physical Properties of Coal (CO-14)
The physical properties of coal, such as color, specific gravity, and hardness, vary considerably. This variance depends on the composition and the nature of preservation of the original plant material that formed the coal, the amount of impurities in the coal, and the amount of time, heat and pressure that has affected the coal since it was first formed. Time, heat and pressure determine the rank of the coal, which is classified according to the increasing amount of carbon as lignite, subbituminous coal, bituminous coal, and anthracite.

Indiana's bituminous coal is black, but elsewhere lignitic and subbituminous coals are various shades of brown and black. Generally the specific gravity of coal (its weight compared with an equal volume of water) varies with rank and with amount of impurities. The higher the rank and the higher the amount of impurities, the higher the specific gravity. Most Indiana coal has a specific gravity of about 1.3. Hardness of coal varies over a broad range, but knowing this range is important in designing equipment for mining, processing and using coal.

Our Hoosier State Beneath Us: Coal