Here's why:

* Sedimentary Origin: Coal starts as sedimentary rock, formed from the accumulation and compression of plant matter over millions of years.

* Metamorphic Transformation: Intense heat and pressure, often associated with geological events like mountain building, transform the sedimentary coal into anthracite. This process changes the coal's structure, composition, and properties.

Anthracite is the highest rank of coal, meaning it has undergone the most intense metamorphism. It has a high carbon content, burns with a very hot, clean flame, and is harder and denser than lower-rank coals.