Everyday Uses of Rocks and Minerals - Shelf 13
When we describe oil and coal as fossil fuels, we mean it: they are produced by the cooking of decomposed plant and animal matter deep in the earth's crust over many millions of years. Fossil fuels are a form of solar power: they are energy from the sun trapped by plants millions of years ago. Coal is simply the remains of woody plants that died in swampy conditions and was cooked down into a solid mass. Large amounts of wood accumulated on earth during the Carboniferous period, 359 to 299 million years ago, because plants evolved wood and no organisms on earth evolved the ability to digest wood for 50 to 60 million years! Think of a world where tree trunks never decompose because there are no microbes that know how to break them down. That's the Carboniferous world that left us with a legacy of coal.
Graphite is elemental carbon, just like diamond. The difference is that diamond forms at extremely high pressures, which cause the carbon atoms to line up in a strong mineral. Graphite is formed under much lower pressures and has a mineral structure that makes it slippery and easy to break. We use it for the 'lead' in pencils because it makes a good, but erasable, mark. We also use it as a powder for lubrication.
Both coal and graphite are composed primarily of carbon.