Diamonds, which are one form of pure carbon, are created from intense heat and pressure that happen deep inside the earth over eons. In modern times, scientists have learned how to recreate this heat and pressure to make artificial diamonds.
Natural diamonds are found in pipe-like structures in the earth called kimberlites. These kimberlites penetrate as much as 100 miles into the earth's crust. The intense heat that creates the diamonds in these kimberlite pipes can be over 2,000 degrees Fahrenheit, and the pressure must be at least 435,113 pounds per square inch. Lower temperatures and pressure produce other forms of carbon such as graphite.
Over millions of years, diamonds arrive at the surface of the earth, transported up through the kimberlites by tectonic forces or by the process of erosion. Because diamond is the hardest natural substance known, the gems retain much of their original shape. They are sometimes found in placer deposits, which are accumulations of sediments laid down by running water that contain enough diamonds or other minerals to make mining them profitable.
Tiny diamonds, called microdiamonds, can also result from meteorite impact. The heat and pressure from the impact is enough to change carbon into diamond. Diamonds can also form in supernovae.