DIRK WIERSMA / SCIENCE PHOTO LIBRARY DIRK WIERSMA / SCIENCE PHOTO LIBRARY
Anthracite coal. Anthracite is the hardest type of coal with the highest carbon content. As it has few impurities, it burns with a clean flame, and so is the preferred coal for use as a fuel. A metamorphic rock, it is formed when sedimentary coal (mined as bituminous coal) is subjected to high temperatures and pressures within the Earth's crust. This sample measures 85 millimetres across.
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