Key Concepts

Diamond and graphite are both composed of carbon. 

The earth’s basic structure consists of the crust, mantle, and core.

The mantle can be divided into three broad layers: the upper mantle, lower mantle, and a transition zone in between.

Mantle convection drives the movement of the earth’s plates. 

Subduction is an essential part of diamond formation.

Most diamond deposits are found on cratons or in the areas surrounding them.

Diamonds form mainly in two types of rocks: peridotite and eclogite. 

Diamond formation requires extreme pressures and temperatures, as well as a source of carbon. 

The conditions for diamond formation exist within mantle keels under cratons. 

The majority of mined diamonds are lithospheric and superdeep.

Not all diamonds form in mantle keels; superdeep diamonds form much deeper within the mantle.

Diamonds form in mantle keels when carbon is available and free to bond with other carbon atoms without oxygen. 

Inclusions can indicate where a diamond formed within the earth.   

CLIPPIR diamonds crystallized from metallic melts in the transition zone or the lower mantle. 

The most common cause of blue color in diamonds is boron. 

Boron in blue diamonds originates from oceanic plates. 

Carbonado is the toughest form of diamond. 

Diamonds were delivered to the surface between 2.5 billion and 20 million years ago. 

Kimberlite and lamproite are the two igneous rocks that transport diamonds to the surface. 

Diamond-bearing kimberlite is much more common than diamond-bearing lamproite. 

Kimberlite tends to occur in the center of cratons, while lamproite occurs at their edges. 

Kimberlite and lamproite form pipe deposits. 

Very few kimberlites or lamproites contain diamonds in economic quantities.