Where Carbon Becomes Diamond

Diamond does not form simply because carbon is exposed to high pressure. If pressure alone were sufficient, diamonds would be far more common.

For natural diamond to grow, at least four sets of conditions must coincide:

• pressure and temperature at which diamond is stable or can be preserved;
• a source of carbon;
• a mobile medium—a fluid or melt—that transports the carbon;
• a chemical reaction that enables nucleation and crystal growth.

Most natural diamonds studied in gemology and geology belong to the lithospheric population. They typically formed in the deep continental mantle, approximately 150–200 kilometers below the surface and most often at temperatures on the order of 900–1300 °C. These are not universal limits: some populations yield broader estimates, while rare sublithospheric or “superdeep” diamonds originate at much greater depths.

The most important distinction A diamond generally did not form in the kimberlite or lamproite that brought it to the surface. The crystal is usually far older than the magma that transported it.

The diamond window in the deep Earth

At Earth’s surface, graphite is the thermodynamically more stable form of pure carbon. Diamond nevertheless does not spontaneously transform into graphite on a human timescale because such a change would require the rearrangement of an enormous number of strong bonds in the crystal lattice. Under surface conditions, diamond is therefore metastable: it is not the most stable phase, but it is kinetically highly persistent.

As pressure increases, the denser diamond structure becomes favored. The graphite–diamond stability boundary also depends on temperature, so there is no single universal depth at which “diamonds begin.”

In cold, thick continental roots, the geotherm can cross into the diamond stability field. In warmer or thinner lithosphere, the same pressure may be accompanied by a temperature that is unfavorable for the long-term preservation of diamond.

[VISUAL 6.1: P–T diagram—the graphite and diamond stability fields and a cold cratonic geotherm]

Cratons and their deep roots

A craton is a very old and long-lived stable part of a continent. Of particular importance for diamonds is its deep root of subcontinental lithospheric mantle, which extends to approximately 200 kilometers or more beneath some cratons.

Such a root may be:

• colder than the surrounding convecting mantle;
• chemically depleted by previous melting;
• relatively buoyant and mechanically stable;
• sufficiently long-lived to preserve diamonds for hundreds of millions or billions of years.

An old craton alone, however, does not guarantee a deposit. A diamond-forming event must have occurred in the appropriate part of the mantle, the crystals must have survived later metasomatic and thermal processes, and a later magma must actually have intersected them and transported them toward the surface.

This is why many kimberlites are either not diamond-bearing at all or do not contain diamonds in the quantity and quality required for economic extraction.

A geotherm and a depth are not the same thing

Formation depth is usually not measured directly. Geologists first use the chemistry of mineral inclusions to estimate pressure and temperature, then convert the pressure into an approximate depth using density and geotherm models.

For that reason, the statements “formed at 5 GPa” and “formed at 160 km” are not equally direct claims. The first is usually the result of a geobarometric model; the second is an additional interpretation that converts that pressure into depth.

This matters when comparing the literature: different calibrations, geotherms, and assumptions can produce slightly different depth ranges even when they describe similar mantle environments.

Two principal lithospheric environments

Mineral inclusions show that most lithospheric diamonds are associated with two major groups of mantle rocks:

• peridotitic;
• eclogitic.

Rarer groups also exist, including websteritic diamonds. The detailed classification of these rocks and their mineral assemblages belongs to Chapter 7.

The peridotitic environment is associated with an ultramafic mantle dominated by olivine and pyroxenes. The eclogitic environment is rich in garnet and omphacitic clinopyroxene and, in many cases, preserves evidence of deeply metamorphosed basaltic material. Some of that material may have a history connected with the subduction of former oceanic lithosphere.

This distinction shows that there is no single “recipe” for natural diamond.

Fluid, metasomatism, and chemical reaction

Diamond is generally misunderstood if it is imagined as a piece of graphite that has merely been compressed in the mantle. Evidence from mineral and fluid inclusions supports a model in which a large proportion of natural diamonds crystallize from carbon-rich fluids or melts that migrate through preexisting rocks.

When such a mobile medium reacts with a rock, it changes the rock’s chemical composition. This process is called metasomatism.

Diamond may form when there is a change in:

• redox conditions;
• fluid composition;
• temperature;
• pressure;
• the acidity or basicity of the system;
• the ratio of dissolved carbon species;
• the composition of the rock with which the fluid reacts.

Chapter 8 examines the types of diamond-forming fluids in detail. Here it is enough to understand the principle: diamond growth is a chemical reaction in the deep mantle, not merely a consequence of pressure.

Carbon can have different origins

The carbon from which a diamond grew need not have the same geological source in every population.

It may be associated with:

• carbon that had long been part of the mantle;
• carbonates recycled by subduction;
• reduced carbon species;
• in some cases, material that had previously participated in the surface carbon cycle.

Carbon isotopes and inclusion chemistry can constrain possible sources, but they rarely permit a simple story such as “this diamond formed from organic carbon.” Such conclusions must always be tied to the specific population and body of evidence.

Redox as a chemical boundary

In the mantle, carbon may be distributed among different species and minerals. Depending on the oxidation state of the system, it may be more stable as carbonate, CO₂, methane, graphite, diamond, or part of other phases.

When a mobile fluid enters a rock with a different redox capacity, the equilibrium may shift toward diamond precipitation. In another environment, that same diamond may begin to dissolve. For this reason, modern models often treat growth as the result of a fluid–rock reaction, rather than simply cooling or decompression.

Growth is episodic

A diamond need not grow continuously from the core to the crystal surface.

Cathodoluminescence images, spectroscopy, and zoning reveal that a single crystal may contain:

• multiple growth zones;
• interruptions in growth;
• zones of partial dissolution;
• a new generation of diamond over an older core;
• chemically distinct phases formed from different fluids.

The “age of a diamond” can therefore be a complex concept. Different parts of the same crystal may record different events. Dating and its limitations are discussed in Chapter 9.

Lithospheric and superdeep diamonds

Lithospheric diamonds make up the great majority of studied natural diamonds. Recent GIA summaries place superdeep diamonds at roughly 1–2% of mined diamonds, but the proportion is poorly constrained and can vary substantially by deposit and sampling.

These rare crystals may originate in the mantle transition zone, between approximately 410 and 660 kilometers, or even in the lower mantle below 660 kilometers. Their inclusions may preserve high-pressure phases or products formed by their retrograde alteration.

Their mineralogy and significance are discussed in Chapter 11. It is important not to turn the label “superdeep” into a conclusion without appropriate confirmation from inclusions, chemistry, or structure.

[VISUAL 6.2: Cross section of Earth—lithospheric diamonds in a cratonic root and sublithospheric diamonds in the transition zone/lower mantle]

Formation and transport are not the same event

A diamond may spend billions of years in the mantle before being entrained by kimberlite or lamproite. The transporting magma then performs an entirely different geological role: it rapidly carries an existing crystal from depth toward the surface.

The diamond is not entirely passive during the journey. It may partially dissolve and resorb, lose its edges, or develop new surface features. This is why the external shape of a rough diamond is not always its original growth form.

Transport is discussed in detail in Chapter 10, and rough morphology in Chapter 13.

What we can and cannot conclude

From formation depth, we cannot automatically infer:

• market quality;
• color;
• clarity;
• a specific mine;
• value;
• the age of an individual stone.

A geological population describes a formation environment. Gemological quality and market value are separate questions.

Chapter summary

• Natural diamond does not form through high pressure alone.
• Most studied natural diamonds belong to the lithospheric population from deep continental roots.
• Typical lithospheric conditions are often approximately 150–200 km and 900–1300 °C, with genuine variation among populations.
• Cold, thick cratonic roots permit the long-term stability and preservation of diamond.
• Peridotitic and eclogitic environments are the two principal lithospheric parageneses.
• A large proportion of natural diamonds crystallize from mobile carbon-rich fluids or melts.
• Metasomatism and redox reactions can initiate diamond growth or dissolution.
• Growth may be episodic, and a single crystal can contain multiple generations.
• Sublithospheric or superdeep diamonds are rare, but they provide direct samples of the much deeper mantle.
• Kimberlite and lamproite are generally transporters, not the rocks in which most diamonds formed.

[VISUAL 6.3: Life cycle of a natural diamond—mantle growth → long-term storage → entrainment by magma → transport → surface]