Kimberlite is a type of igneous rock. It forms from the cooling of magma like other igneous rocks, but is unique because it forms over 100 miles (150 kilometers) beneath the Earth’s surface and, in a matter of hours, travels to the top. It is also unique because it is one of two types of rock, along with lamproite, in which diamonds are found. In diamond mining, non-oxidized kimberlite is referred to as blue ground. Kimberlite, colored yellow from limonite, is called yellow ground. The name kimberlite comes from Kimberley, South Africa, where in the 1800s a large quantity of kimberlite containing diamonds was discovered, creating a diamond rush and the Big Hole. Kimberlite is what geologists call “ultrabasic” rock because it does not contain quartz or feldspar, which are the main rock-forming minerals. It is instead composed mainly of olivine, sometimes mica, serpentine and calcite. Olivine is made up of magnesium, iron and silica, which gives kimberlite an olive or brownish-green color, but can be colored differently by other minerals, like in blue and yellow ground.
The depth at which kimberlite forms, the mantle, is rich in peridotite containing olivine. Temperature and pressure melt some of the peridotite, and if volatile gases are present at the time of formation, this will propel the kimberlite magma upward. As the kimberlite magma rises to regions of the earth with lower temperatures and pressures, it begins to crystallize while the gases expand. The gases put increased pressure on the surrounding rock, which is them incorporated into the kimberlite magma. Close to the surface the kimberlite becomes explosive, rising at rates of 1,200 feet (400 meters) per second. The rapid movement gives kimberlite its porphyritic texture as it rips through other rock. Porphyritic textures are made up of large rounded crystals surrounded by a fine-grained matrix.
Kimberlite intrusions have been clustered into 10 time periods throughout history between 1,600 million and 55 million years ago. Kansas kimberlite, for example, intruded during the Cretaceous period, 90 million years ago. The intrusions form pipe-like structures as the magma cools. The kimberlite pipes of the Kimberley region are vaguely funnel-shaped, being wider nearer to the surface and getting thinner and thinner at increased depths. The majority of the world’s kimberlite has been found in South Africa, where over 3,000 pipes of kimberlite have been found. 200 pipes are known in North America, most near the border between Colorado and Wyoming. Of all the known sources of kimberlite, only around 1,000 contain diamonds, and only between 50 and 60 of those contain enough diamonds to be viably mined.