Note that only the cratonic lithospheric keel is cold enough at high enough pressures to retain diamonds. Most of the mantle is within the field of diamond stability.The crust, which is normally too thin (usually less than 40 km thick) to lie within this field, can do so only if it has been thickened by the geologic processes related to .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 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. Archean cratons in South Africa have yielded gem diamonds such as these specimens from the GIA Museum’s Oppenheimer Student Collection. Since then, there have been significant advances in the analysis of diamonds and their mineral inclusions, in the understanding of diamond-forming fluids in the mantle, and in the relationship of diamonds to the deep geology of the continents and the convecting mantle. It has been more than two decades since diamond ages have proven to be up to billions of years older than their host magmas of kimberlite or lamproite.
But it is also of importance to the practicing gemologist, since these are fundamental questions that a wearer of a beautiful diamond might ask.The most prized specimens for research are flawed with visible inclusions (figure 2), for these carry actual samples of mantle minerals from depths as great as 800 km beneath the surface.Diamond provides the perfect container for mantle minerals, isolating them from the high pressure and temperature reactions within the earth for geologic time scales.Terms listed in the glossary are italicized on their first use in the text. Carbon is widely dissolved in the earth’s silicate minerals at part-per-million levels and lower.But whenever carbon occurs as a free species, diamonds have the potential to form.Yet diamonds are very rare because the mantle has a relatively low abundance of carbon. Earth is special among the planets in that it has two crustal types, continental and oceanic, that sit at two very different heights, approximately 840 meters above and 3,840 meters below sea level on average.