Diamonds

a long string of superlatives to describe its properties, its technological and commercial importance.
Diamond, the king of gems, is at the heart of the most lucrative part of the gem industry,
with an unmatched combination of brilliance, fire, Hardness. Natural diamonds are probably the oldest and deepest-sourced objects we will ever touch, and provide direct information about the mantle.

elegant crystal structure (FIG. 1), in which each carbon atom is bonded to four other atoms in a tetrahedral arrangement, yields a strong rigid framework. Combining this structural arrangement, which coincides with the hybrid sp3 orbitals of carbon, with the unmatched strength of the C–C bond, explains most of diamond’s properties.

of graphite (2.20 g cm-3), the other common polymorph
of carbon, is a clear indication that diamond is a
high-pressure mineral, formed mostly in Earth’s interior.
Thus, diamond is a key indicator and recorder of events
deep within our planet, in part because its extreme strength
and refractory nature permits it to survive exhumation to
Earth’s surface and subsequent weathering (another aspect
is the extraordinary volcanic style of kimberlites and lamproites,
which act as express elevators to raise diamonds
quickly from depth, but that is a different story). Moreover,
inclusions captured in a diamond growing in the mantle
are protected by its adamantine embrace, so diamonds
have become our “space missions” to inner Earth, providing
our most important samples for understanding the
chemistry of the deep mantle.

them, researchers have discovered the association
of diamond with peridotite and eclogite assemblages
from the roots of ancient cratons. More recently, transition-
zone and lower-mantle signature minerals have been
identified. The contribution by Stachel, Brey, and Harris
reviews the status of these, the deepest samples of Earth
that we have at our finger tips. Diamonds, while essentially
pure carbon, allow us to investigate their carbon source
through isotopic analysis of C and the minor contained N.
Cartigny presents the available isotopic data and shows
how diamonds reveal the hallmarks of primitive Earth,
recycled crustal sources, and crystallization processes.

is that of the nanometer-sized diamonds in meteorites. Meteoritic diamonds are hardly new, since they were described in the Canyon Diablo iron meteorite in 1891. On the other hand, diamonds in the Nova Urei (Ringwood 1960; Carter et al. 1964) and Kenna (Berkley et al. 1976) ureilites formed by shock on the meteorite parent body. Searching for the most primitive materials and reservoirs of noble gases in primitive meteorites, led Ed Anders and colleagues to seek the last moieties in meteorites that could not be dissolved by aggressive acid or base—diamond, graphite, and silicon carbide. Huss reviews the results of research on these “nanodiamonds” and their possible origin in supernovae prior to the formation of our solar system.