Carbon Carbon is distributed very widely in nature. It is found in abundance in the sun, stars, comets, and atmospheres of most planets. The atmosphere of Mars contains 96 % CO2. Carbon is found free in nature in three different forms: amorphous, graphite, and diamond. The two common forms are graphite and diamond. Graphite is found in pencil lead and is one of the softest known materials while diamond is one of the hardest known materials. Carbon, as microscopic diamonds, is found in some meteorites. Natural diamonds are found in ancient volcanic "pipes" such as found in South Africa. Diamonds are also recovered from the ocean floor off the Cape of Good Hope. Carbon is also found combined in materials such as limestone. All living tissues contain carbon compounds, called organic compounds Carbon is present as carbon dioxide in the atmosphere and dissolved in all natural waters Carbon dioxide plays a vital role in photosynthesis and respiration in living organisms. Carbon is a component of rocks as carbonates of calcium (limestone), magnesium, and iron. Coal, petroleum, and natural gas are chiefly hydrocarbons. Carbon is unique among the elements in the vast number of variety of compounds it can form. The study of carbon and its compounds forms its own subsection of chemistry, called organic chemistry.