The diamonds form as minerals within rocks of the upper mantle called eclogite and peridotite. The olivine rich peridotite is called dunite, and the pyroxene rich rock is called websterite. The rock harzburgite contains up to 40 percent orthopyroxene and 60 to 90 percent olivine and is thought to be the most common host rock for diamonds. Often these xenoliths of the upper mantle peridotites or eclogites contain diamonds. A rock composed of angular fragments is referred to as a breccia.
Most rocks are millions to billions of years old.
Recent dating of inclusions in diamonds from kimberlite pipes, mainly in South Africa and Australia, indicate that diamonds formed as early as 3,300 million years ago to as late a 990 million years ago, an extended period of the earth's history. The kimberlite, which carried the diamonds to the surface and is the present host rock, was intruded only about 100 million years ago. Kimberlites ascend rapidly to the earth's surface at rates thought to be on the order of 10 to 30 kilometers per hour. There is usually no evidence of any substantial thermal reaction with the surrounding country rock. In the near surface environment, velocities may increase to serveral hundreds of kilometers per hour because of gas expansion in the ascending magma and reaction with water.