Superdeep Diamonds

Most natural gem diamonds belong to the lithospheric population and formed in the deep, cold roots of old continents, typically approximately 150–200 kilometers below the surface. A small but scientifically exceptional group comes from much deeper—from the sublithospheric mantle, the mantle transition zone, and the uppermost lower mantle.

These crystals are called sublithospheric or superdeep diamonds. Recent GIA summaries place them at roughly 1–2% of mined diamonds, but the estimate is poorly constrained and can vary substantially by locality and sampling method. The number should therefore not be treated as a precise global proportion.

The most important lesson Superdeep origin is not demonstrated by a diamond’s size, color, clarity, or type. The evidence comes from mineral phases, their chemical assemblages, and other features that require pressures greater than those found in the continental lithosphere.

The depth zones that diamonds can sample

Four approximate zones are sufficient for this chapter:

• continental lithosphere—the principal environment of most natural diamonds;
• sublithospheric upper mantle—below the base of the lithosphere to the discontinuity at approximately 410 km;
• mantle transition zone—approximately 410–660 km;
• lower mantle—beginning at approximately 660 km and extending much deeper.

Most confirmed superdeep diamonds do not come from near the core. Their estimated depths are generally on the order of several hundred kilometers, often approximately 300–800 km.

[VISUAL 11.1: Cross section of Earth—the lithospheric population, 410 km, the transition zone, 660 km, and the uppermost lower mantle]

A high-pressure phase is not always preserved to the surface

A mineral stable at extreme pressure may become unstable during decompression. Within a diamond, it may:

• remain partly preserved because of residual pressure;
• transform into a low-pressure structure;
• break down into multiple phases;
• develop fractures or a reaction rim;
• preserve a chemical composition from which the former high-pressure phase can be reconstructed.

Interpretation is therefore often more complex than asking, “Which mineral do we see today?” It may be more important to reconstruct what that inclusion was under deep-Earth conditions.

Majoritic garnet

As pressure increases, garnet can incorporate increasing amounts of components that would reside in pyroxene at lower pressures. Such high-pressure garnet is described as majoritic garnet.

Its chemistry can aid a geobarometric estimate of depth, but majorite does not signify one single depth. Interpretation depends on:

• garnet composition;
• the bulk composition of the rock;
• temperature;
• experimental calibration;
• possible retrograde alteration.

The term “majorite” must therefore not be converted automatically into a number of kilometers without chemical and petrogenetic context.

Ringwoodite and water in the transition zone

Ringwoodite is a high-pressure form of olivine composition that is stable in the lower part of the mantle transition zone, at approximately 520–660 km depth. Its presence as an inclusion in a diamond provides strong evidence of origin from that deep region.

The best-known example from Juína, Brazil, contained approximately 1.4 wt.% water in the form of hydrogen incorporated into the crystal structure. This finding shows that at least part of the transition zone may be substantially hydrated.

It does not show that a liquid ocean exists deep within Earth or that the entire transition zone is uniformly saturated with water.

Evidence and its limit Hydrous ringwoodite demonstrates both the capacity to store water and the local presence of water in the transition zone. The global amount of water requires a much broader geophysical and mineral-physics model.

Bridgmanite and ferropericlase

Below approximately 660 km, the mineralogy changes. Bridgmanite, a magnesium–iron silicate with a perovskite structure, is the dominant phase of the lower mantle, together with ferropericlase and other minerals.

The difficulty is that bridgmanite is generally not preserved in its original form at low pressure. Diamonds may contain retrograde products, such as orthopyroxene/enstatite whose chemistry points to former bridgmanite.

Enstatite alone is not proof of the lower mantle. A stronger conclusion requires appropriate chemistry, an assemblage with other phases, texture, and pressure retained in the inclusion.

The same applies to ferropericlase: it is highly important in lower-mantle assemblages, but an isolated phase without context must not become a universal depth marker.

Davemaoite: an important correction of scientific status

Davemaoite is CaSiO₃ perovskite, a phase expected in the lower mantle that can concentrate certain elements important to Earth’s geochemistry and thermal evolution.

Its stability as a distinct lower-mantle phase now has strong experimental support. Experiments published in 2025 showed that the solubility of calcium in bridgmanite is insufficient to eliminate davemaoite from typical lower-mantle assemblages.

A separate issue is the specific natural inclusion in a diamond reported in 2021 and the interpretation of its K- and Na-rich composition. That specimen has been the subject of published scientific debate. Experimental work from 2024 showed that davemaoite formed from subducted eclogitic material at relevant deep-mantle conditions incorporates only small amounts of K and Na. The authors therefore concluded that the anomalously K- and Na-rich composition of the reported inclusion was not simply inherited from deeply subducted oceanic crust, further supporting earlier criticism that this specific specimen may not be representative lower-mantle davemaoite.

It is therefore necessary to distinguish:

1. the existence and stability of davemaoite in the lower mantle—well supported experimentally;
2. the identity of an individual natural inclusion—which may remain disputed;
3. the geochemical history and significance of its trace elements—a separate interpretive problem.

Breyite and retrograde Ca-silicates

During decompression, Ca-silicate inclusions may transform into phases such as breyite or into more complex multiphase assemblages. Such phases can be important evidence of deep origin, but the final interpretation must be based on bulk composition and texture.

In the appropriate context, breyite may point to an original high-pressure Ca-silicate and superdeep origin. Experiments have shown, however, that breyite can also form directly under upper-mantle conditions, at approximately 6–8 GPa, in certain hydrous carbonate–silicate systems. The mineral name alone is therefore insufficient either for the claim “deeper than 360 km” or for precise depth determination; a stronger conclusion requires residual pressure, texture, chemistry, and/or a multiphase assemblage.

Jeffbenite: why a depth gauge must be reassessed

Jeffbenite was long treated as a mineral associated with superdeep diamonds and as a possible indicator of the transition zone or uppermost lower mantle.

Thermodynamic work published in 2023 showed that the Mg end-member of jeffbenite may be stable at much lower pressures, approximately 2–4 GPa over the temperature range considered. This strongly supports the possibility that at least some natural jeffbenite is a retrograde phase formed during decompression, rather than an original high-pressure mineral.

The editorial rule is therefore simple:

Jeffbenite alone is not proof of superdeep origin.

Its significance derives from an assemblage of other minerals, bulk chemistry, and geological context.

CLIPPIR: large and unusually pure superdeep diamonds

A distinct group of large, relatively pure, and strongly resorbed rough diamonds is described by the acronym CLIPPIR—Cullinan-like, Large, Inclusion-Poor, Pure, Irregular and Resorbed.

They typically show:

• large masses;
• very little nitrogen, often type IIa;
• few mineral inclusions;
• irregular and strongly resorbed rough morphology;
• occasional metallic Fe–Ni–C–S inclusions.

Mineral assemblages and metallic inclusions in studied CLIPPIR diamonds point to formation at approximately 360–750 km depth. An iron- and nickel-rich metallic liquid containing carbon and sulfur shows that some of these diamonds form under the strongly reducing conditions of the deep mantle.

It is important not to reverse the conclusion: a large, colorless, type IIa diamond is not automatically a CLIPPIR diamond. Mineralogical or other depth confirmation is required.

[VISUAL 11.2: Principal superdeep indicators—majoritic garnet, ringwoodite, former bridgmanite + ferropericlase, Ca-silicate assemblages, and metallic Fe–Ni–C–S inclusions]

Subduction, water, and carbon

Superdeep diamonds show that surface and shallow geological material can be transported hundreds of kilometers into the planet’s interior.

Subduction may transport:

• water bound in minerals;
• carbonates;
• organic and inorganic carbon;
• boron;
• elements from altered oceanic crust and serpentinized mantle.

As pressure, temperature, and redox state change, this material can participate in the formation of fluids, melts, and diamond. There is no single universal superdeep “recipe”: different populations point to different growth media, ranging from carbonate systems to highly reduced metallic systems.

How superdeep origin is demonstrated

The most important methods include:

• microscopy and textural analysis;
• Raman spectroscopy;
• X-ray diffraction;
• FTIR;
• chemical analysis by electron microprobe or other microanalytical methods;
• measurement of residual pressure;
• for special problems, synchrotron and other advanced methods.

One method rarely resolves everything. The strongest conclusion arises when structure, chemistry, assemblage, and pressure agree with one another.

A standard laboratory grading report generally does not determine formation depth. D color, type IIa, or high clarity may be consistent with some superdeep populations, but they are not evidence of depth.

Chapter summary

• Superdeep diamonds form below the continental lithosphere, most often at depths on the order of several hundred kilometers.
• Their global proportion is commonly estimated at roughly 1–2%, but it is not well constrained.
• The mantle transition zone lies approximately between 410 and 660 km.
• High-pressure inclusions and their retrograde products are the principal evidence of depth.
• Majoritic garnet can serve as a geobarometer, but it does not indicate one single depth.
• Ringwoodite is a strong indicator of the lower part of the transition zone.
• Hydrous ringwoodite demonstrates local deep storage of water, not a global liquid ocean.
• At surface pressure, bridgmanite is often recognized through retrograde products and chemistry.
• Davemaoite is a well-supported lower-mantle phase, but individual natural inclusions may remain the subject of debate.
• Jeffbenite alone is no longer acceptable as a reliable high-pressure depth gauge.
• CLIPPIR diamonds form a distinct superdeep population of large, pure, and strongly resorbed diamonds.
• Subduction links the deep cycles of water, carbon, boron, and other elements with the growth of superdeep diamonds.

[VISUAL 11.3: Confidence levels in depth interpretation—preserved high-pressure phase → multiphase assemblage → retrograde reconstruction → uncertain individual indication]