Diamonds, the hardest substance found in nature, are made of carbon, which is the same material that is found in the much softer graphite that fills our pencils. What differentiates diamonds from graphite is that the carbon atoms in diamonds are much more tightly packed and more strongly bonded than they are in graphite. Diamonds require extremely high temperatures and high pressure to form. Natural diamonds are created at depths of at least 100 kilometers beneath the surface of the earth in the earth's mantle, the layer of partially molten material between the earth's core and its outer crust. It is there that the extremely high temperatures of up to 2,100°C and intense pressure between 50,000 and a million times the atmospheric pressure on the earth's surface needed to create diamonds exist.
Though natural diamonds are created at great depths below the surface of the earth, they are brought much closer to the surface by means of geological formations called diamond pipes. Formed of cylindrical intrusions of molten material from deep within the earth that are somewhat similar to the geological formations that supply molten lava to active volcanoes, diamond pipes allow molten material containing diamonds from the earth's mantle to penetrate the earth's crust. The resemblance of diamond pipes to the feeder pipes that serve as conduits for molten lava from the earth's mantle to active volcanoes on the surface is not unexpected: diamond pipes are generally considered by geologists to be the remnants of ancient volcanoes that erupted between 80 million and 120 million years ago. Over the eons of time that have passed since the volcanoes were active, the forces of erosion have destroyed visible evidence of volcanic cones from the surface of the earth, yet the diamond-filled pipes beneath the surface of the earth that resulted from this long-past volcanic activity still remain.
The rock in which diamonds are found is a unique rock that is of interest not only to miners for the diamonds that it contains but also to geologists for the wealth of information that it holds. This rock, called kimberlite after the city of Kimberley, which is in the heart of the region that is home to extraordinarily productive diamond pipes, has different characteristics beneath the surface of the earth and on its surface. The kimberlite in the upper part of the pipes and on the surface of the earth has been exposed to weathering and is a softer yellowed substance called yellow ground by miners; deeper in the pipes, where the kimberlite has been protected from the effects of weathering, the kimberlite is a harder blue substance that is called blue ground by miners. While clearly the importance of kimberlite to a miner is that it is known to hold diamonds, which can be extracted from it, the importance of kimberlite to a geologist is that it contains substances that originated in a layer of molten material in the earth's mantle, deep beneath the earth's crust. Through the study of kimberlite, geologists can find clues about the inner workings of depths of the earth that are otherwise inaccessible to them.
