Diamonds are formed by exposure of carbon bearing materials to high pressure and temperature. On Earth, the formation of diamonds is possible because there are regions deep within the Earth that are at a high enough pressure and temperature that the formation of diamonds is thermodynamically favorable. Under continental crust, diamonds form starting at depths of about 90 miles, where pressure is roughly 5 Gigapascals and the temperature is around 2,200 degrees Fahrenheit.
Diamond formation under oceanic crust takes place at greater depths because of higher temperatures, which require higher pressure for diamond formation. There are many things that geologists do not understand about diamond formation, including how fast diamonds grow. “Is it like an instantaneous process, or does it take two days or 10 years or a hundred years?” That issue ultimately relates to questions about the history of the Earth and the geological processes within the planet. Diamonds that have come to the Earth’s surface are generally considered to be very old, ranging from under 1 billion to 3.3 billion years old.