Kimberlite, Diamond-Bearing: Kimberlite is the most common raw ore from which gem-grade diamonds are extracted. Kimberite is typically found in the remains of a rare, and relatively small, explosive type of volcano known as a Maar. Kimberite is an igneous rock generally believed to have been formed deep within the Earth's mantle; from between 80 and 250 miles below the surface. Sizeable Kimberite deposits have been found, and are commercially exploited, in South Africa, Russia, Australia, Canada, and the United States. The mining of Kimberite for diamonds began in the 1870s in South Africa and will likely continue for centuries to come.