Deposits of Diamond Pipes

Diamonds are formed in the rock peridotite and eclogite, 150 km or more under the Earth’s surface, where extreme temperature and pressure make diamond, not graphite, the stable form of the element carbon. Diamonds are brought nearer the surface by volcanic pipes and dykes of kimberlite eruption carries xenoliths, or rock fragments, of diamond-bearing eclogite or peridotite up through the Precambrian basement rock and overlying rock formations.   

Mushroom-shaped kimberlite pipes, the most common type, are typically found in clusters. Some may contain economic quantities of diamonds, some may contain only a few diamonds and others may not be diamondiferous at all. There are a number of “indicator” minerals, such as certain types of garnets, spinels and ilmenite, that can reveal the presence of kimberlites. 

Diamonds have also been found in dykes of lamprophyre, an ultramafic rock similar to kimberlite.

Commercial quantities of lode, or hard-rock, diamonds have been found in several locations in sub-Saharan Africa, Siberian Russia, Australia and, most recently, Canada’s Northwestern Territories.