The earliest diamonds were found in the 4th century BC in India. The youngest of these diamond deposits were formed some 900 million years ago. The diamond mines in India were depleted in the 18th century, after which a quest to find new mines started. Read on from www.brilliance.com/history-of-diamonds.
Most natural diamonds are formed at high temperature and pressure at depths of 140 to 190 kilometers in the Earth's mantle. Carbon-containing minerals provide the carbon source, and the growth occurs over periods from 1 billion to 3.3 billion years. Diamonds are brought close to the Earth?s surface through deep volcanic eruptions by a magma, which cools into igneous rocks known as kimberlites and lamproites. Diamonds can also be produced synthetically in a high-pressure high-temperature process which approximately simulates the conditions in the Earth's mantle. An alternative, and completely different growth technique is chemical vapor deposition. http://en.wikipedia.org/wiki/Diamond