Kimberlite = Diamonds Diamonds are the most amazing of gems. Just as amazing, however, is how natural diamonds reach Earth’s surface. Diamonds are formed 150 to 700 km deep in Earth, and are then carried upward in a rare volcanic eruption of a kimberlite magma. Man has never witnessed such an event, and the eruption of this magma is thought to be the most rapid and violent type of volcanic eruption on Earth. Luckily, since diamond is the hardest mineral, it can usually survive such rough handling. This delivery system in the form of volcanic transport only adds to the mystique and value of natural diamond.
There are two main magma types that carry natural diamonds to the surface. These magmas crystallize on cooling into volcanic rocks known as kimberlite and lamproite (see box A). Kimberlite is by far the dominant type of eruption to bring diamonds to Earth’s surface (figure 1). Although diamond is only an accidental passenger and not actually created by the kimberlite, a basic understanding of kimberlites helps us understand the setting for most natural diamond formation in the mantle.