The physical properties of carbon vary dramatically depending on the form the element takes. A diamond, which is transparent and the hardest mineral found in nature, is a very pure form of carbon. Curiously, so is graphite, which is opaque and black and one of the softest naturally occurring minerals. The number of different forms carbon can take makes any simple description impossible.
Carbon is the 15th most abundant element found in the earth's crust. It is also the fourth most abundant element by mass in the universe. Its chemical structure allows it to bond with itself and many other chemical elements to form nearly 10 million different compounds, more than any other element. Carbon bonds with oxygen to form carbon dioxide, carbon monoxide and many other less common compounds. It combines with hydrogen to form gases such as methane, liquids such as petroleum and solids such as coal. Due to carbon's ability to bond with itself and form long chains, there are more than 100,000 different known varieties of hydrocarbon. Carbon is also one of the most essential elements of all life on earth. The study of organic chemistry is the study of carbon compounds. It is found in plants, animals and minerals.