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Occurrence

In alluvial deposits the harder and heavier diamonds survive when parent rock weathers and is worn away. They are mined from the original rocks only in Siberia, South Africa, Australia, and in Arkansas. They usually occur in a basic plutonic rock in cylindrical, more or less vertical, volcanic plugs known as "pipes." In Canada many suitably located pipes (and glacial deposit finds) indicate that there is a possibility of economic diamond-bearing formations in the north. Alluvial localities, perhaps reweathered with their prime source long gone, are numerous. Sporadic diamonds are found in gold placers in the eastern U.S. and in California. In recent decades Siberia has become an important source. Brazil, New Guinea, India, Namibia, and other African states have many localities, though none has proved notably large or abundant.