Mineral Inclusions

A diamond can carry to the surface a mineral sample from the deep Earth that, without the protection of the diamond host over geologic time, would have been reshaped, reacted, or disappeared. This is precisely why mineral inclusions have value far beyond their effect on the clarity of a gem. They can help reconstruct the host rock, pressure and temperature, mantle chemistry, the timing of particular geologic events, and, in rare cases, processes in the transition zone or lower mantle.

But the same inclusion does not represent the same kind of information in gemological and geologic language. On a clarity plot it may simply be marked as a crystal. In a mineralogical analysis, the same feature may be olivine, garnet, clinopyroxene, chromite, sulfide, or a high-pressure phase. Appearance guides examination; identity requires evidence.

“Crystal” Is Not a Mineral Diagnosis

In clarity grading, crystal is an observational term for a sufficiently large solid inclusion within a diamond. It does not determine its full mineral species, chemical composition, paragenesis, age, or geographic origin.

The following levels of claim must therefore be separated:

1. a crystalline inclusion is visible under magnification;
2. it is marked as a crystal on the laboratory diagram;
3. spectroscopy or diffraction confirms the mineral species;
4. chemical composition determines the paragenetic group;
5. textural, crystallographic, geochemical, or isotopic evidence is used to assess its relationship to diamond growth;
6. a geologic interpretation may ultimately be derived from a broader body of data.

Each step requires more evidence than the preceding one.

Protogenetic, Syngenetic, and Epigenetic

A mineral may exist before the growing diamond traps it, may form during the same broader growth episode, or may enter the system after the host’s main growth. These relationships are traditionally described as protogenetic, syngenetic, and epigenetic.

An important modern qualification is that a shape imposed on the inclusion by the diamond is not in itself proof of syngenesis. The same applies to epitaxial orientation. Studies of olivine, garnet, pyroxene, periclase, and other inclusions have shown that preexisting crystals can be partially dissolved, reshaped, and then enclosed by a growing diamond. A 2024 study further challenged the use of epitaxy alone as a criterion for proving that the inclusion and diamond nucleated at the same time.

This does not mean that a mineral inclusion is useless for geochronology. A protogenetic mineral may undergo reequilibration or isotopic resetting during a diamond-forming event. Interpretation must therefore not be reduced to the formula “older mineral = an older date unrelated to the diamond.”

[VISUAL 41.1: Temporal relationship of inclusion and host—protogenetic, syngenetic, and epigenetic scenarios, with a warning that morphology and epitaxy alone are not proof]

Paragenesis: the Assemblage Matters More Than Color

Paragenesis describes the mineral and chemical context in which an inclusion is interpreted. For lithospheric diamonds, the three most important groups are peridotitic, eclogitic, and websteritic/pyroxenitic.

Group	Representative phases	Interpretation
Peridotitic	olivine, orthopyroxene, chromium-rich garnet, clinopyroxene, chromite, sulfides	association with an ultramafic mantle environment
Eclogitic	garnet, omphacitic clinopyroxene, kyanite, coesite or its products, rutile, sulfides	association with a mafic, eclogitic assemblage
Websteritic/pyroxenitic	garnet, clinopyroxene, and orthopyroxene in transitional combinations	a pyroxenitic or reaction/metasomatic environment
Sublithospheric/superdeep	high-pressure or retrogressed phases, Ca silicates, ferropericlase, majoritic assemblages, metallic Fe–Ni–C–S phases	possible connection with the transition zone or lower mantle, subject to phase-specific verification

The table is a guide. A single mineral name may not be sufficient for a final paragenesis because chemical composition is often decisive.

Peridotitic Inclusions

Olivine is one of the most common upper-mantle minerals and a common peridotitic inclusion. In diamonds it is usually magnesium-rich. Its color may be colorless, pale green, or different depending on composition and optical conditions, but visual impression is not identification.

Orthopyroxene, often enstatite-rich, and clinopyroxene belong to pyroxene groups whose compositions must be analyzed. A green appearance is not sufficient to declare an inclusion chrome diopside or enstatite.

Chromium-rich garnet may be red, purple, or pink, but garnet is a broad mineral group. Cr, Ca, Mg, Fe, and other constituent elements—not color alone—matter for petrogenetic interpretation.

Chromite and other spinels may be dark and highly reflective. Sulfide inclusions are particularly important because, in the appropriate context, they may enable Re–Os geochronology, although their present composition may result from cooling and exsolution from an original high-temperature phase.

Eclogitic Inclusions

The eclogitic group typically includes garnet and omphacitic clinopyroxene, while kyanite, rutile, sulfides, and phases related to the high-pressure SiO₂ system may also occur.

Eclogitic garnet is not the same as peridotitic chromium-rich garnet. A pyroxene may have an omphacitic composition, and the mineral association and chemistry together provide a stronger diagnosis than an individual phase.

Coesite is a high-pressure polymorph of SiO₂. During decompression, it may transform into quartz or leave a polyphase assemblage. Today’s Raman or diffraction pattern may therefore represent the product of retrograde transformation, not necessarily the original phase at the time of entrapment.

Websteritic and Transitional Groups

Nature need not conform to neat textbook categories. Websteritic or pyroxenitic associations may contain garnet, orthopyroxene, and clinopyroxene in combinations that lie chemically between classic peridotitic and eclogitic fields.

Such inclusions may record fluid–rock reaction, metasomatism, reservoir mixing, or complex mantle evolution. A “transitional” composition should therefore not automatically be treated as a classification error.

[VISUAL 41.2: Peridotitic, eclogitic, and websteritic paragenesis—mineral assemblage and chemistry instead of color]

Superdeep Inclusions: Evidence Must Be Phase-Specific

Sublithospheric diamonds may contain mineral assemblages associated with pressures greater than those typical of the cratonic lithosphere. Important examples in the literature include majoritic assemblages, ringwoodite, ferropericlase, high-pressure Ca silicates, products of bridgmanite, and metallic Fe–Ni–C–S phases.

However, these names are not a list of universal “depth gauges.” Three reasons are crucial:

• a phase may be a retrograde product of a mineral stable at higher pressure;
• chemical composition may matter more than the name itself;
• the same mineral or a related structure may have several possible petrogenetic pathways.

It is particularly important that jeffbenite is no longer used as a simple stand-alone marker of extreme depth. More recent thermodynamic interpretation shows that its Mg endmember may be stable at shallower conditions and supports the possibility of retrograde formation. Similarly, discussions of natural Ca-silicate inclusions must distinguish a phase stable in the lower mantle from the mineral measured today after decompression-related transformation.

The full geodynamic context remains in Chapter 11.

[VISUAL 41.3: Lithospheric and superdeep mineral inclusions—the original high-pressure phase, retrograde product, and confidence level of the interpretation]

The Mineral Seen Today May Not Be the Mineral That Was Trapped

Diamond is an exceptionally strong container, but an inclusion inside it is not completely isolated from physics. During ascent and cooling, the following may occur:

• phase transitions;
• exsolution;
• volume change;
• formation of several daughter phases;
• fractures in the surrounding diamond;
• chemical redistribution within the inclusion.

This problem is especially pronounced for superdeep inclusions. The scientific task is often not “to name what is inside today,” but to reconstruct which phase was stable under the conditions of entrapment.

Color, Habit, and Relief: Useful Clues, Not Proof

A red mineral may be garnet, but not necessarily. A green mineral may be olivine or pyroxene, but color is insufficient. A black inclusion may be graphite, sulfide, oxide, a metallic phase, or a combination of several materials.

Under the microscope, record:

• color and transparency;
• shape;
• surface relief;
• metallic or nonmetallic luster;
• fractures and pressure halo;
• reflections;
• relationship to growth zones;
• whether the inclusion is fully enclosed or opened by polishing.

This is a morphological description, not a final mineralogical identification.

How a Mineral Is Identified

Different methods answer different questions.

Raman spectroscopy can nondestructively identify numerous crystalline phases through the host diamond and is often the first choice.

XRD, including synchrotron diffraction in advanced research, provides direct insight into crystal structure.

EPMA and SEM-EDS determine chemical composition when the inclusion is accessible or properly prepared. More sensitive microanalytical techniques may be used for trace elements.

Micro-CT and other 3D methods help determine volume, position, fractures, and the relationship with the host, but they do not replace mineral identification.

The choice of method should follow the question. A rare stone should not be subjected to destructive analysis if the answer can be obtained nondestructively.

[VISUAL 41.4: Analytical sequence—microscopy → Raman/XRD → chemistry → petrogenetic interpretation]

Residual Pressure Is Not a Simple Depth Scale

An inclusion trapped at high pressure may remain compressed within rigid diamond under surface conditions. This may be expressed as a shift in Raman bands or altered crystal-lattice parameters.

Elastic geobarometry was developed on this basis. But converting residual pressure into entrapment conditions requires models of host and inclusion elasticity, thermal history, geometry, possible fractures, and equilibrium assumptions. “Raman shift = depth” is therefore not an acceptable formula.

Enclosed and Surface-Opened Inclusions

Cutting may intersect an inclusion that was previously fully enclosed. Opening it alters the analytical and conservation context:

• residual pressure may be lost;
• volatile components may escape;
• cleaning or polishing compounds may enter;
• the inclusion surface may oxidize or react;
• the risk of contamination during chemical analysis increases.

A scientifically valuable inclusion is therefore documented in its existing state before sampling or recutting.

Paragenesis Is Not Geographic Origin

A peridotitic or eclogitic inclusion may narrow the geologic context, but it does not by itself identify a mine or country. Similar mantle assemblages occur beneath different cratons.

Geographic origin requires a separate evidentiary system. A mineral inclusion may form part of a population profile, but it is not a geographic passport for an individual stone.

Practical Protocol

Professional examination of a mineral inclusion proceeds from least invasive to more invasive:

1. document the entire diamond and the inclusion’s position;
2. describe color, shape, relief, reflections, and fractures microscopically;
3. check whether the inclusion is enclosed or reaches the surface;
4. record its relationship to growth zones if visible;
5. perform nondestructive spectroscopic or diffraction identification when possible;
6. only then decide whether chemical microanalysis is required;
7. derive paragenesis from mineralogy and chemistry, not color;
8. treat the temporal relationship to the diamond as a separate question;
9. infer depth, age, and origin only with appropriate independent evidence.

Chapter Summary

• Crystal in clarity language is not a complete mineral diagnosis.
• Mineral identity requires structural and/or chemical evidence.
• Protogenetic, syngenetic, and epigenetic relationships describe the timing of an inclusion relative to the host.
• A diamond-imposed shape and epitaxy are not stand-alone proof of syngenesis.
• Peridotitic, eclogitic, and websteritic groups are determined by assemblage and chemistry, not color.
• Superdeep phases must be interpreted together with retrograde transformations.
• Jeffbenite must not be used as a universal marker of extreme depth.
• Raman, XRD, and chemical microanalysis answer different questions.
• Residual pressure may be useful, but it is not a simple depth scale.
• Opening an inclusion during cutting can change its condition and analytical value.
• Paragenesis is not the same as geographic origin.
• The scientific value of an inclusion and its effect on a diamond’s market value are two separate axes.