Kimberlites--narrow, pipelike fissures through continental crust are unusual ultramafic rocks that apparently have been explosively emplaced from deep within the mantle.
They are serpentinized mica peridotites that contain a variety of high pressure minerals, including diamonds.
The most notable are found in South Africa, Tanzania, Angola, and Siberia. Kimberlites are often found associated with alkaline basaltic rocks and carbonatites.
Most kimberlite pipes were emplaced during the Cretaceous Period (135 to 65 million years ago), although one of the largest in South Africa, the Premier, dates from 1,150 million years ago.
The kimberlite pipe at the famous Kimberley diamond mine in South Africa is the archetype, typifying their narrow surface area (approximately 40,000 sq m /450,000 sq ft) and great depth (from more than 1,000 m/3,500 ft below the present land surface to the limit of mining).
At the time of the eruption, the Kimberley pipe extended an estimated additional 1,400 m (4,600 ft) through now eroded layers of overlying sediments and formed a deep, narrow funnel of debris and kimberlite magma.
The famous peridotite of pyroxene olivine mixture, now largely altered to serpentine, in South Africa, a mix of diamonds and the "Blue ground".