The earth’s upper surface is composed of rigid, lithospheric plates of crustal rock (too stiff to flow on geologic time scales, yet stiff enough to break and cause earthquakes) underlain by mantle rock.Surface deformation, volcanic activity, and earthquakes occur more readily at the margins of plates than at their interior.
It is hoped that this article will give the gemologist a ready way to convey how nature first created the rough diamonds.
For the reader who is unfamiliar with geologic terms, a glossary is presented at the end of the article.
Even low elemental concentrations and minute features in diamond can now be analyzed using instruments with higher sensitivity and resolution.
As a result, study combining the inclusion and its diamond host is a powerful tool for geologic research, which itself has improved our understanding of diamond formation. These photos show inclusions of silicate minerals in natural diamond whose background reflectivity has been enhanced by faceting: almandine (left), magnified 10×; pyrope (center), magnified 40×; and diopside (right), magnified 30×. The purpose of this article is to describe our current understanding of where, how, when, and why natural diamonds have been formed.
The continental crust is old—up to four billion years old.
Its oldest parts, the ancient continental nuclei, or , are isolated in the interior of the continent by belts of successively younger continental crust (figure 4).Only a subset of these localities are rich enough to be mined for diamonds.The crustal age/craton basemap is from Pearson and Wittig (2008). (2009), Harte (2010), Harte and Richardson (2011), Tappert and Tappert (2011), Dobrzhinetskaya (2012), and the authors.Research into natural diamonds (figure 1) has emerged over the last two decades as one of the keys to understanding the deep earth.Analytical advances, improved geologic knowledge, and the emergence of new diamond-producing regions (such as the Slave craton of Canada) have all contributed to this change.Carbon in the earth can occur in oxidized forms, such as when bound with oxygen in CO, or in reduced forms such as diamond, graphite, or bound with hydrogen in methane and other organic molecules.