Igneous Origin of Diamonds

Diamonds are a rare occurrence on the surface of the planet because it takes extremely hot and high pressure conditions to create them. Physical and chemical conditions where diamonds form only exist in the mantle, nearly 70 miles down or more. In that environment in the upper mantle, diamonds may be a common mineral! It takes incredible events, nothing that has ever been witnessed in historic times, to bring diamonds to the surface.
Kimberlite-Diamond-mantle-rock

Diamond deposits around the world (that have any economic significance) are associated with volcanic features called 
diatremes.  A diatreme is a long, vertical pipe formed when gas-filled magma forces its way through the crust to explosively erupt at the surface. Kimberlite a special kind of intrusive igneous rock associated with some diatremes that sometimes contain diamonds, typical coarse grained an bluish in color.    

Diamond-bearing kimberlite pipes are diatremes that originate in the mantle.
   
Diamonds are xenoliths carried up from deep sources in the mantle, and often occur in association with other gem minerals including garnet, spinel and diopside. Most "economically significant" diamond deposits occur in ancient rocks (Precambrian age), but have been discovered on all continents. Because diamonds are so hard, they survive torturously-long histories, recycled through sedimentary and metamorphic environments without being destroyed. As a result they have been found almost everywhere as very rare, isolated discoveries. Diamonds of microscopic size have been discovered in meteorites and asteroid impact sites, and some metamorphic rocks. They are most extensively mined from kimberlite pipes or from alluvial gravels derived downstream from diamond source areas. It should be noted that most diamonds are not of gem quality, but those that are not are used for industrial purposes.

Credits to Phil Stoffer at geologycafe.com
Text and figures are used with permission.