Diamonds are not native to Earth’s surface. Instead they form at high temperatures and pressures that occur in Earth’s mantle about 100 miles below Earth’s surface.

How do diamonds form? A detailed article that explains the four sources of diamonds found at Earth’s surface.

Most of the diamonds that have been discovered were delivered to Earth’s surface by deep-source volcanic eruptions. These eruptions begin in the mantle, and on their way up they tear out pieces of mantle rock and deliver them to Earth’s surface without melting. These blocks from the mantle are known as xenoliths. They contain diamonds that were formed at the high temperature and pressure conditions of the mantle.

People produce diamonds by mining the rock that contains the xenoliths or by mining the soils and sediments that formed as the diamond-bearing rocks weathered away.

Some diamonds are thought to form in the high-temperature/pressure conditions of subduction zones or asteroid impact sites. Some are delivered to Earth in meteorites. No commercial diamond mines have been developed in deposits with these origins.