How do diamonds form?

Most natural diamonds formed in the Earth mantle, at depths of around 150 kilometres or more, where temperatures and pressures are high enough to force carbon atoms into the tight, rigid structure of diamond. This process took place over very long periods, often billions of years ago. The diamonds were later carried rapidly upward by deep volcanic eruptions that produced a rock called kimberlite, which is where miners find them today. The extreme conditions are essential; the same carbon under milder conditions would form graphite instead. Diamonds can also be created artificially by mimicking these conditions in the laboratory.