Rocks That Preserve Diamonds

A diamond may be small, but its inclusion can be a direct sample of rock from a depth that drilling cannot reach.

To extract geological information from such a sample, we must distinguish three things:

1. a mineral—an individual crystalline phase;
2. a rock or paragenesis—an assemblage of minerals that together describe a geological environment;
3. a transport rock—kimberlite, lamproite, or another material that later carried the diamond.

The vague expression “host rock” can easily blur the distinction among these three levels. This book will therefore refer more precisely to the growth rock, mantle host, or transport rock, depending on what the evidence actually supports.

Paragenesis: a mineral association

Paragenesis describes a mineral assemblage associated with a common geological environment. Among lithospheric diamonds, the three most important groups are:

• peridotitic;
• eclogitic;
• websteritic.

Large datasets of diamonds with identified inclusions show the approximate dominance of the peridotitic and eclogitic groups, with a much smaller proportion of websteritic material. Such percentages depend on sampling and locality and must not be presented as a precise global ratio for all diamonds in trade.

Key rule A single inclusion color or shape is not a paragenesis. The mineral must be identified, and the geological conclusion must be built from its chemistry, its relationship with other phases, and its context.

Peridotite: the dominant rock of the upper mantle

Peridotite is a group of ultramafic rocks generally dominated by olivine, with varying amounts of orthopyroxene, clinopyroxene, and either garnet or spinel, depending on pressure and composition.

Of particular importance in the diamond-bearing mantle are:

• harzburgite—olivine + orthopyroxene, with very little clinopyroxene;
• lherzolite—olivine + orthopyroxene + a significant amount of clinopyroxene;
• dunite—a rock exceptionally rich in olivine.

Harzburgitic sources often represent strongly depleted mantle that lost readily fusible components in the distant geological past. Such mantle may be cold, buoyant, and stable over long periods—precisely the properties that matter for preserving a cratonic root.

Lherzolite is chemically more “fertile” because it retains more clinopyroxene and components that would enter a melt more readily during partial melting. This does not mean that lherzolite is geologically younger or that it necessarily produces higher-quality diamonds. Harzburgitic and lherzolitic describe rock and mineral parageneses, not commercial categories of stone.

Typical peridotitic inclusions include:

• olivine;
• chromium pyrope;
• chromite;
• orthopyroxene;
• chromium-bearing clinopyroxene;
• sulfides.

The mineral name alone, however, is not always sufficient. Its chemical composition often determines the paragenesis to which it belongs.

Eclogite: a high-pressure basaltic rock

Eclogite is a high-pressure rock dominated by garnet and omphacitic clinopyroxene. Many mantle eclogites are associated with basaltic or gabbroic protoliths metamorphosed at high pressure.

In diamonds, an eclogitic paragenesis may be indicated by:

• omphacite;
• garnet of an appropriate composition;
• kyanite;
• coesite or products associated with former coesite;
• sulfides.

Some eclogitic systems carry isotopic and chemical evidence of recycled oceanic lithosphere. This is important evidence for the deep recycling of material, but eclogite is not automatic proof of organic carbon or of a subduction history for every individual diamond.

Coesite is especially interesting because it is a high-pressure polymorph of SiO₂. During transport toward the surface, it may transform into quartz, which can produce fractures and characteristic textures. When the structure is preserved, or when retrograde transformation is properly demonstrated, such an inclusion can provide strong evidence of a high-pressure history.

[VISUAL 7.1: Peridotitic, eclogitic, and websteritic parageneses—typical mineral assemblages]

Websterite and transitional groups

Websterite is a pyroxenite rich in orthopyroxene and clinopyroxene, with less olivine than typical peridotite. Websteritic diamonds and their inclusions are rarer and may represent complex transitional or metasomatically altered environments.

Because of chemical overlap with peridotitic and eclogitic systems, websteritic classification often requires more than one mineral and a more detailed chemical analysis.

An inclusion may be older than the diamond

Traditionally, a well-formed inclusion inside a diamond was often described as syngenetic, as though it had crystallized at the same time as the diamond. More recent research shows that morphology alone is not sufficient.

An inclusion may be:

• protogenetic—it existed before the diamond;
• syngenetic—it formed during the same growth event;
• epigenetic—it entered or formed after the main growth phase, for example along a fracture.

Even a protogenetic mineral may chemically or isotopically re-equilibrate with a fluid during a diamond-forming event. The relationship between an inclusion and its diamond must therefore be established through texture, crystallography, chemistry, and isotopes, rather than shape alone.

How a mineral is confirmed

Microscopy provides an initial clue, but color and luster are often insufficient for a final identification.

Research methods include:

• Raman spectroscopy—for a mineral’s structural “fingerprint”;
• electron microprobe analysis—for quantitative chemical composition;
• X-ray diffraction—for crystal structure;
• SEM-EDS and other electron-beam methods—for microtextures and chemistry;
• synchrotron techniques—for extremely small or high-pressure phases.

Detailed analytical workflows belong to Chapters 41–42 and 68–72. The important boundary here is that the gemological term “crystal” is not a mineral identification.

Geothermobarometry: minerals as gauges of conditions

Certain mineral pairs and chemical compositions respond to pressure and temperature. These relationships can be used to estimate the conditions under which a rock or mineral assemblage was in equilibrium.

Such methods are known as geothermometers and geobarometers.

The result is not a direct depth-gauge reading. It depends on:

• model calibration;
• an assumption of equilibrium;
• the precision of the chemical analysis;
• later re-equilibration;
• the selected mineral pair;
• the assumed geotherm.

An estimate such as “approximately 5 GPa and 1100 °C” must therefore not be turned into a claim that a crystal grew at one exact depth without uncertainty.

A particular problem arises if a mineral pair was not in equilibrium at the same time, or if one mineral later underwent diffusion and changed composition. The best estimates therefore combine several independent thermometers, barometers, textural relationships, and the local geological setting.

Xenoliths and xenocrysts

Kimberlite does not transport only individual diamonds. It can tear away larger fragments of mantle rock—xenoliths—or individual crystals—xenocrysts.

Xenoliths are exceptionally important because they permit comparison of:

• the whole rock;
• mineral relationships;
• texture;
• chemistry;
• pressure and temperature;
• metasomatic changes.

A diamond in a xenolith provides especially strong context because the physical connection between crystal and rock is preserved. Even then, however, it must be determined whether the rock remained a closed and representative system during transport.

Indicator minerals are not proof of diamond

Exploration uses mantle indicator minerals that can survive weathering and sediment transport. Certain garnets, chromites, pyroxenes, and other minerals may indicate the presence of a deep source favorable for diamonds.

An indicator mineral does not automatically mean:

• that kimberlite exists at the immediate location;
• that it is diamond-bearing;
• that the grade is economic;
• that the diamonds are of gemological quality.

It is a clue in the exploration chain, not final proof.

Paragenesis is not a geographical passport

Peridotitic and eclogitic inclusions occur on several continents. Mineral paragenesis alone therefore generally cannot identify a mine or country.

Geographic origin requires a documented chain of custody or a specifically validated matching system. The limits of geographic inference are discussed in Chapters 5, 79, and 80.

Chapter summary

• A mineral and a rock are not the same: a rock is an assemblage of multiple mineral phases.
• Paragenesis describes minerals associated with a common geological environment.
• Peridotitic and eclogitic groups dominate among lithospheric diamonds; the websteritic group is rarer.
• Harzburgite and lherzolite are different types of peridotite and must not be reduced to a single “mantle stone.”
• Eclogite is a high-pressure rock rich in garnet and omphacite.
• An inclusion may be protogenetic, syngenetic, or epigenetic.
• The shape, color, and luster of an inclusion are insufficient for mineral identification.
• Raman spectroscopy, electron microprobe analysis, and other methods connect appearance with mineralogical and chemical confirmation.
• Geothermobarometry provides a model-based estimate of P–T conditions, not an absolute depth without uncertainty.
• Xenoliths provide the broader context of an entire mantle rock.
• Indicator minerals assist exploration but are not proof of an economic deposit.
• Paragenesis alone does not determine a diamond’s geographic origin.

[VISUAL 7.2: Chain of evidence—microscopic appearance → mineral identification → chemistry → paragenesis → geological model]