Unusual Natural Forms

Diamond is not always a single transparent crystal. Nature also creates aggregates in which thousands or millions of tiny diamond grains are joined into one body.

This group includes the names carbonado, ballas, bort/boart, and framesite. They are not four strictly separate mineral species. Some names come from historical trade, some from mining practice, and some from descriptive mineralogy, so their boundaries occasionally overlap in the literature.

The most important lesson For unusual natural diamond aggregates, microstructure, grain size, porosity, and geological context are more important than the trade name alone.

Monocrystalline and polycrystalline diamond

In a monocrystalline diamond, most of the stone belongs to one continuous crystal lattice. In polycrystalline diamond, the body consists of a large number of differently oriented grains.

Grain boundaries can deflect and arrest fractures. Polycrystalline diamond can therefore be very tough, even though the individual grains have the same fundamental diamond structure as a monocrystalline stone.

Polycrystallinity is not automatically an advantage. Mechanical behavior depends on:

• grain size;
• bonding between grains;
• porosity;
• graphite and other phases;
• fractures and voids.

Carbonado

Carbonado is a porous, dark, strongly intergrown natural polycrystalline diamond. Classic carbonado in the strict sense is best documented from Brazil and the Central African Republic, principally from secondary sedimentary deposits. Its primary source rock has not been found.

It typically shows:

• black, gray, or brown color;
• extremely small, randomly oriented diamond grains;
• many pores and channels;
• localized shiny or “glassy” patinas;
• secondary minerals in open pores.

Porosity reduces its apparent density relative to compact monocrystalline diamond. At the same time, the complex network of grains hinders the propagation of a single continuous fracture, which explains carbonado’s historical industrial value.

[VISUAL 12.1: Cross sections of monocrystalline diamond and carbonado—a continuous lattice versus a porous polycrystalline aggregate]

Secondary material in pores

A mineral found in an open pore in carbonado need not be part of its original formation environment. Over geological time, quartz, iron oxides, phosphates, clay minerals, and other materials from the sedimentary environment may enter the pores.

Research must therefore distinguish:

• material enclosed during growth;
• later intergranular material;
• secondary filling of an open pore.

This distinction is critical to the debate over carbonado’s origin.

The age and origin of carbonado

Very old radiogenic ages, approximately 2.6–3.8 billion years, have been published for carbonado. Such dating is not the same as classic dating of mineral inclusions in many other diamonds, so its geological significance requires caution.

The relatively well-established facts are:

• carbonado is natural polycrystalline diamond;
• classic samples from Brazil and the Central African Republic show strong similarities to one another;
• the primary source is unknown;
• common mantle inclusions typical of kimberlitic diamonds are largely absent;
• the microstructure, porosity, and isotopic characteristics do not fit neatly into the standard model for monocrystalline kimberlitic diamond.

Proposed models include deep-mantle growth and sintering, subduction, an impact process, radiation, and an extraterrestrial scenario. A 2024 study also proposed a more specific subduction model in which very old shungite-like carbonaceous material could be a precursor. This is a new working hypothesis, not a consensus. No model is generally accepted as the final solution.

An extraterrestrial origin, like the other individual models, therefore belongs in the category of hypothesis, not confirmed fact.

Sergio and large masses

The best-known large carbonado, Sergio, found in Brazil in 1905, had a mass of approximately 3,167 ct. Its enormous mass shows that polycrystalline aggregates can reach dimensions greater than those of the largest historically known monocrystalline rough diamonds.

This does not mean that carbonado is “harder than diamond.” It is diamond material; its distinctive nature lies primarily in the combination of the hardness of its individual grains and its polycrystalline resistance to fracture propagation.

Ballas

Ballas is a historical name most often used for approximately spherical or rounded diamond aggregates and multidomain crystals, often with a radial internal structure.

The name is not fully standardized. Some sources use it for clearly polycrystalline material, while others use it for complex multidomain growth. Technical writing should therefore state the actual texture instead of merely calling a specimen “ballas.”

Bort or boart

Bort is primarily a broad historical and industrial term for natural diamond material unsuitable for standard gem cutting.

It may include:

• heavily included monocrystals;
• irregular fragments;
• aggregates;
• small industrial grains.

Bort is neither a fake diamond nor a distinct chemical species. It is a functional category whose meaning has changed with technology.

Framesite and diamondite

Framesite is used for certain finer-grained polycrystalline diamond aggregates associated with kimberlitic environments. Unlike classic carbonado, such aggregates may contain mantle silicates, oxides, and sulfides and have a clearer relationship with the known geological diamond system.

The name diamondite is also used in the literature for polycrystalline diamond rocks or aggregates, sometimes more broadly than framesite. The boundaries among these terms are not entirely uniform, so the name should always be accompanied by information on:

• grain size;
• porosity;
• mineral phases;
• texture;
• geological context.

Carbonado is not a synonym for black diamond

A black appearance can result from very different causes.

A black diamond in jewelry may be:

• a natural monocrystalline diamond with abundant dark inclusions;
• a natural diamond containing graphite and fractures;
• a treated diamond whose appearance has been artificially darkened;
• carbonado or another polycrystalline aggregate.

Color must therefore not be used as a mineralogical classification.

Identification

A thermal conductivity tester may confirm high thermal conductivity, but its result can be uneven on a porous, heterogeneous aggregate and is insufficient for full characterization.

More rigorous analysis uses:

• microscopy;
• Raman spectroscopy;
• X-ray diffraction;
• CT or micro-CT;
• FTIR and other spectroscopic methods.

Modern industry now meets most technical needs once served by natural bort and carbonado with controlled synthetic diamond and polycrystalline diamond materials. Natural origin alone is not a technical advantage.

Chapter summary

• Natural diamond may be monocrystalline or polycrystalline.
• Microstructure strongly influences fracture behavior, porosity, and industrial usability.
• Carbonado is a porous, strongly intergrown natural polycrystalline diamond.
• Classic carbonado is best documented from Brazil and the Central African Republic, and its primary source is unknown.
• Material in open pores may be secondary and must not automatically be used to reconstruct formation.
• Published very old ages for carbonado require a methodological caveat.
• The origin of carbonado remains unresolved; the extraterrestrial model is only one hypothesis.
• Ballas is primarily a spherulitic or multidomain diamond form with a historically variable definition.
• Bort is a broad functional or trade category of genuine diamond material.
• Framesite/diamondite describe polycrystalline diamond aggregates whose actual microstructure should be stated alongside the name.
• Black diamond is not a synonym for carbonado.
• Natural polycrystalline diamond today has no automatic technical advantage over engineered synthetic PCD materials.

[VISUAL 12.2: Comparison of carbonado, ballas, bort, and framesite—appearance, microstructure, porosity, and geological context]