Limestone, coral, and oyster shells are largely calcium carbonate. Marble, dolomite, and magnesite also contain calcium, magnesium, and carbon. Coal, petroleum, natural gas, and oil shale are mainly compounds of carbon and hydrogen derived from plant and animal sources deposited in the Earth millions of years ago and subjected to high pressure. These deposits were once a part of what is called the carbon cycle, a dynamic system of change still occurring. Through photosynthesis, plants use sunlight to convert carbon dioxide from the air and convert water from the soil into plant tissues such as cellulose and into an energy source such as sugar.
Plants release oxygen into the air as the carbohydrates sugar and cellulose are synthesized.
Animals eat the plants, breathe in oxygen from the air and oxidize the carbohydrates, or use them as fuel, which releases energy to the animal. The cycle repeats itself endlessly. The third form, fullerene, is unstable in comparison to these two forms and is thus not found widely. A diamond, no matter what the size, may be considered to be a single molecule of carbon atoms, each joined to four other carbons in regular tetrahedrons, or triangular prisms. The crystal structure is called a face- centered cubic lattice.