Anthracite is a type of coal classified as a metamorphic rock. It forms from bituminous coal that has undergone intense metamorphism involving high pressure and heat, typically in areas with folded and deformed sedimentary rocks such as mountain flanks. This process drives out volatile substances, increasing the carbon content to over 90%, making anthracite hard, dense, and shiny with a semi-metallic luster and a conchoidal fracture. Unlike typical sedimentary coals, anthracite has undergone changes that align it with metamorphic rocks due to the transformation under tectonic pressures rather than simple sedimentation. It burns cleanly with little smoke and ignites with difficulty. Anthracite marks a transition stage chemically between bituminous coal and graphite but does not fully reach graphite in nature without extreme conditions. It is the hardest form of coal and used for heating and industrial purposes.