Comprised solely of the element carbon, diamonds are formed in the upper mantle of the earth's lithosphere where specific pressure and temperature conditions are favourable for diamond growth and preservation. Referred to as the "diamond stability field", these conditions are limited to temperatures of 900 to 1200 degrees Celsius and typically occur from 150 to 200 km beneath the earth's surface. Other evidence indicates that certain "ultra deep" diamonds may originate at depths of 700 km or greater. Researchers postulate that the diamonds were crystallized in the mantle over one billion years ago, but only brought to surface by younger events.
Diamonds are generally of two types: peridotitic or "P-type" diamonds and eclogitic or "E-type" diamonds. P-type diamonds crystallize within peridotitic source rocks in the upper mantle. These rocks are enriched in chrome and are dominantly composed of olivine, orthopyroxene, clinopyroxene, chromite and pyrope garnet. E-type diamonds crystallize within eclogitic source rocks in the mantle. Eclogites are essentially biminerallic rocks composed of pyrope-almandine garnet and omphacitic clinopyroxene. In addition to providing an environment for the generation of diamonds, these peridotitic and eclogitic rocks are also the source of the indicator minerals that typically occur in kimberlites and are the object of indicator mineral sampling programs.