Diamond can crystallize throughout the mantle below about 150 km and while it exists metastably in the crust, it is the only material sampling the ‘very deep’ mantle to depths exceeding 800 km. Diamond has been intensively studied over the last 40 years to provide extraordinary information on Earth’s deep interior. Work on diamonds has pinpointed the initiation of subduction, tracked the transfer of material through the mantle transition zone, recorded tFrosthe timing and source of ingress of fluids to the continental lithosphere, preserved carbonatitic fluids that trigger deep mantle melting, captured the redox state of the mantle, and provided samples of primordial carbon and noble gasses. As a result of the formation of diamond from a mobile C-bearing phase in the mantle, typically a ‘C-O-H bearing fluid or melt’, diamond has the remarkable ability to track carbon mobility in the deep mantle, as well as mantle redox state and can preserve mantle minerals as inclusions. Thus, diamond occupies a unique position in discussion of the igneous and metamorphic aspects of the Earth’s carbon cycle. This talk will review observations and research on naturally occurring diamonds as they apply to diamond formation, carbon bearing species in the mantle, the role of carbon during mantle melting, the geologic history of the mantle, and the difficulties to be unlocked in future studies.