Feature Gems & Gemology, Summer 2017, Vol. 53, No. 2

Carbonado Diamond: A Review of Properties and Origin


Separation of Congo–São Francisco island in southwest Rodinia into two cratonic blocks
Figure 1. Left: The Congo–São Francisco island in southwest Rodinia, at approximately 1.1 billion years ago (Ga), is the only known site of carbonado that was originally deposited ca. 3.8 Ga on a possibly even smaller cratonic island. Continental masses in Rodinia are underlain by ancient cratons approximately 2.5 to 4.0 Ga. Green zones are 1.1 Ga mountain belts, and the red dots are granite intrusions (Torsvik, 2003).Right: Separation of the microcontinent into two cratonic blocks, now Brazil and the Central African Republic, took place during the breakup of Gondwanaland about 180 million years ago.

ABSTRACT

Carbonado diamond is found only in Brazil and the Central African Republic. These unusual diamond aggregates are strongly bonded and porous, with melt-like glassy patinas unlike any conventional diamond from kimberlites-lamproites, crustal collisional settings, or meteorite impact. Nearly two centuries after carbonado’s discovery, a primary host rock compatible with the origin of conventional diamond at high temperatures and pressures has yet to be identified. Models for its genesis are far-reaching and range from terrestrial subduction to cosmic sources.

Discovered in 1841 in Brazil, carbonado was named by Portuguese diamond prospectors for its resemblance to charcoal (Leonardos, 1937; Dominguez, 1996). Carbonado was found later in the Central African Republic. These two localities, now separated by the Atlantic Ocean and situated on the São Francisco and the Congo cratons, respectively, previously shared a common geological setting for more than a billion years (De Waele et al., 2008) on the supercontinent of Rodinia (figure 1) and its precursor Nuna, also known as Columbia.

Carbonado was prized by the French as a superior polishing material. It was used for drilling during the construction of the Panama Canal and formed part of the U.S. strategic mineral stockpile as recently as 1990. At the height of alluvial mining in Brazil (1850–1870), some 70,000 carats were produced by an estimated 30,000 artisanal miners (Svisero, 1995). A conservative estimate of the recovery from Brazil and the Central African Republic is approximately 2 metric tons (Haggerty, 2014). Four of the five largest diamonds reported from Brazil, ranging in weight from 726 to 3,167 ct, are carbonado (Svisero, 1995). The largest of the five, the Sergio, recovered in 1905, is 61 ct heavier than the largest single-crystal diamond ever reported (the 3,106 ct Cullinan rough).

While earlier investigations of carbonado focused on physical and chemical properties and synthesis, more recent studies have introduced dating techniques, high-resolution microscopy, spectroscopy, and an emphasis on origin (see Haggerty, 2014, for a more comprehensive view). The present study offers a detailed examination of about 800 carbonados from Brazil and the Central African Republic (figure 2), ranging from <1 to 730 ct. These samples showed no significant differences in their texture, superficial appearance, and physical and chemical properties (Haggerty, 2014). This article describes the unusual textural features of carbonado, namely their pores and the presence of glassy diamond as a surface patina, with the aim of assessing the origin of carbonado.

Chapada Diamantina, Brazil
Figure 2. A: Scene from Chapada Diamantina, the carbonado site in Bahia, Brazil. B: Boulder of Tombodor conglomerate, the carbonado host rock. C: Polished conglomerate surface in a streambed. D: Artisanal mining of Brazilian alluvial carbonado. Photos by Robert Weldon (A and C) and Stephen E. Haggerty (B and D).

CHARACTERISTICS OF CARBONADO

Carbonado is typically found in five major size categories: >200 ct, 75–95 ct, 25–35 ct, 8–15 ct, and 0.25–1.25 ct (see figure 5 of Haggerty, 2014). Sand-sized particles (<1 mm) also occur, and melon-size objects larger than the Sergio are reported but unconfirmed (M. Ozwaldo, pers. comm., 1996). Carbonados are typically equidimensional (in millimeter to centimeter sizes), although some are elongated (figure 3); they are seldom rounded.

Carbonado samples from Central African Republic and Brazil
Figure 3. The carbonados in the left (118 ct) and center (16.2–52.2 ct) photos are from the Central African Republic, and those on the right (10.8–15.1 ct) are from Brazil. Note the high density of pores, some of which are filled at the surface by crustal infiltrates, and the metallic luster of the glassy melt-like patinas. Photos by Orasa Weldon. GIA Collection numbers 40108–40119; gift of Stephen Haggerty.

Carbonado is opaque, composed of randomly oriented diamond crystallites that impede light refraction and increase absorption. Color varies from black and putty gray to shades of brown (figure 3), deep purple to pink, rusty red, and the occasional olive green. Pores (figure 4), an unusual glassy patina (figure 5), highly irregular surfaces (figures 6 and 7), and polycrystallinity (figures 8 and 9) distinguish carbonado from conventional diamonds.

Open pores in carbonado
Figure 4. Open pores in carbonado (first five photos) and pores covered by a surface patina of nanodiamond approximately 5 µm thick (lower right). Photos by Stephen E. Haggerty.
Melt-like patinas and flow ornamentation on carbonado
Figure 5. Typical melt-like patinas and flow ornamentation on carbonado. Photos by Stephen E. Haggerty.
Melt marbles and flow patterns on carbonado
Figure 6. Melt marbles and flow patterns on carbonado. Photos by Stephen E. Haggerty.
Slickenside patterns and melting of vesicular carbonado
Figure 7. Slickenside patterns and melting of vesicular carbonado with later pits and cratering. The melt layer is about 20 µm thick. Photos by Stephen E. Haggerty.
Phenocrystic diamond cubes and twinned diamond clusters in carbonado
Figure 8. Brightly reflecting phenocrystic diamond cubes (top row) and twinned diamond clusters (circled) in carbonado. SEM images are black and white. Photomicrographs by Stephen E. Haggerty; SEM images by Sven P. Holbik.
SEM images of euhedral diamonds in pores of carbonado
Figure 9. SEM images of euhedral diamonds displaying a variety of morphologies in parallel growth typical of vapor-deposited clusters in the open-space pores of carbonado. Images by Sven P. Holbik.

Porosity. As the porosity of an object increases, its apparent density decreases, because the voids take up more and more of the volume. In carbonado, the number of exposed micro­diamond cutting points increases with porosity. This was a sought-after property that made carbonado more expensive by weight than diamond at the turn of the twentieth century (Haggerty, 2014). Densities as low as 2.8 g/cm3 and as high as 3.45 g/cm3, with most around 3.05 g/cm3 (Trueb and De Wys, 1969; Haggerty, 2014), are in contrast to gem diamond at 3.52 g/cm3. Calculated pore concentrations vary between 5% and 15% in volume. The pores persist into the interior of the carbonado and are either spherical or oblate. Some are inferred to be interconnected (Ketcham and Koeberl, 2013), but the material’s permeability is very low because the pores are free of infiltrating hydro­thermal precipitates that abound in surface pores (again, see figure 4). The spherical pores in carbonado are unlike those in other polycrystalline diamond such as framesite, where the open spaces are at adjoining crystal faces and the shapes are irregularly polyhedral. In other polycrystalline diamonds, the open spaces are microns in width and either radial (in non-gem-quality ballas) or parallel (in fibrous cubes).

Patina. In carbonado, patina surfaces are pervasive (figure 5). Pores in contact with surface patinas are reduced in size, and at 50× magnification they can no longer be distinguished. Glass-like in appearance and similar to synthetic carbon glass (de Heer et al., 2005), these veneers may be dimpled or furrowed, with mounds and flow structures (figures 5 and 6). These textures are akin to those seen in melts in volcanic rocks or in slags from metal processing. But in carbonado, the veneers are diamond that appear to have formed directly from the underlying porous substrates, although diamond coating at a later time is also possible. Contact boundaries between pore-present and pore-absent surfaces are poorly defined, except in cases where patina crusts have splintered off where the contact is sharp, as seen in the lower-right images of figure 6 and in figure 7. Secondary pits and microcraters are pervasive and, in many cases, younger than the patina (figure 7). While pores tend to have sharp outlines (figure 4), craters are rounded with bubbly surfaces or rimmed by smooth ridges (again, see figures 6 and 7). The evidence of flow in both types of voids implies differences in origin. Solid melt marbles are typical. Microcraters, free of ornamentation, grade into texturally soft plastic walls (figures 5 and 6). Slickensides, the striated surfaces known to form on rocks that have been forced to slide along a fracture surface at high pressure as in a fault (figure 7), are of interest because these could only have developed on frictional contact with a body whose hardness was equivalent to another diamond. On the other hand, the patina itself may represent frictional melting (e.g., de Heer et al., 2005; Mitchell et al., 2016; Shumilova et al., 2016a,b; Shiell et al., 2016). Standard diamond testers that measure thermal conductivity give a sharp response to glassy diamond surfaces, less so to the ridges and mounds. The pore-rich surfaces are distinctly sluggish and erratic in response, possibly due to crystal discontinuities of microdiamond grain boundaries.

PHYSICAL AND CHEMICAL PROPERTIES

Octahedra, dodecahedra, tetrahexahedra, and fibrous cubes, all typical of conventional diamond (e.g., Orlov, 1977), are not observed in carbonado. Polycrystalline cubes measuring 5 to approximately 20 µm are common. Encased in very fine diamond (<1–5 µm), the matrix is tightly fused with angular interstices and rounded pores (figure 8). Scanning electron microscopy (SEM) images illustrate the distribution of diamond cleavage surfaces, hopper crystals, skeletal crystallites, re-entrant intergrowths, and layers in single crystals in the open-space pores of carbonado (figures 8 and 9). Trueb and De Wys (1969) and Petrovsky et al. (2010) suggest that the closest analogy to carbonado textures is in synthetically compressed nanodiamond aggregates. Because these structures are found in pores, a more reasonable comparison is with vapor deposition of diamond. The preferred crystal habit of these diamonds is cuboidal, either as single solid cubes or as interpenetrating twins on [111] that follow the fluorite twin law (figure 8). The solid cubes are colorless and, although fine grained, appear to be translucent. Diamond cubes and cuboctahedra are routinely synthesized in metallic catalysts at high pressure and temperature (Burns and Davies, 1992) or by chemical vapor deposition (CVD) under high vacuum and at plasma temperatures (Sato and Kamo, 1992).

X-ray diffraction (XRD) data on crushed carbonado grains are similar to conventional diamonds. Hardness is also similar, but there are data indicating that carbonado is slightly harder (Haggerty, 2014). Its toughness and tenacity, stemming from the random orientation of microdiamonds, are clearly superior to monocrystalline gem diamond, to the point that carbonado can only be cut by lasers.

Yet another unusual feature of carbonado is the presence of an exotic array of metals (Fe, Ni, Cr, and Ti), metal alloys (Fe-Ni, Fe-Cr, Ni-Cr, and W-Fe-Cr-V), and very unusual minerals, specifically moissanite (SiC) and osbornite (TiN). These phases occur as primary intergranular inclusions or as crystal-controlled oriented intergrowths. They are only stable at the very low oxidation states (Gorshkov et al., 1996; De et al., 1998; Makeev et al., 2002; Jones et al., 2003) that would occur deep within Earth’s mantle or other reducing environments such as outer space. By contrast, surface pores and fractures are filled by secondary, low-temperature minerals such as quartz and highly oxidized magnetite, goethite, florencite, and goyazite (Trueb and Butterman, 1969), typical of a more oxidized terrestrial surface growth environment.

Relative to mantle-derived diamonds, carbonado is isotopically light, with δ13C = –24 to –31‰ (Ozima et al., 1991; Shelkov et al., 1997; De et al., 2001). Nitrogen concentrations are low (~20 to 500 ppmw), and δ15N ranges from –3.6 to 12.8‰ with an average of 3.7‰ (Shelkov et al., 1997; Vicenzi and Heaney, 2001; Yokochi et al., 2008). The coupled isotopic distribution of C and N shows that the compositional field for carbonado is distinctly different from that of conventional diamonds (figure 10).

Paired stable isotope plot of C vs. N for conventional diamond and carbonado
Figure 10. A paired stable isotope plot of C vs. N for conventional diamond (top) and
carbonado (bottom). The compositional separation shows that carbonado and deep
Earth diamonds are unrelated. Open symbols are for eclogitic diamonds from
Kimberley, South Africa; filled symbols are for diamonds from Jwaneng, Botswana;
both show extreme variations. The fields for peridotitic (typical inclusions are olivine,
clinopyroxene, and orthopyroxene), eclogitic (garnet and clinopyroxene), and fibrous
diamonds are from a global database. Data for conventional diamonds are from
Cartigny et al. (1998).

Figure 11 shows photoluminescence (PL) spectra of carbonado, which are similar to those of irradiated and heated CVD diamond (Clark et al., 1992). The characteristic peaks at 1.945 eV and 2.156 eV are attributed to nitrogen vacancy (NV) defects in type Ib diamonds. Wang et al. (2009) report a substantial amount of nonaggregated N in type Ib diamonds with H2 and H3 defects. Hydrogen-containing defects (H1) and NV defects are also reported by Nadolinny et al. (2003).

PL spectra of carbonado and CVD diamond
Figure 11. PL spectra illustrating the similarity between carbonado (A and B) and
CVD diamond (C) and CVD diamond that has been heated to 1000°C (D). Modified
from Clark et al. (1992).

Cathodoluminescence of large (approximately 200 µm) monocrystals of diamond in carbonado exhibit orange and green tones (Magee and Taylor, 1999; De et al., 2001; Yokochi et al., 2008). However, blue luminescence in large diamonds, embedded in an orange luminescent matrix of submicron diamond, are also reported (Rondeau et al., 2008). The range in colors is attributed to various N-V (nitrogen-vacancy) defects.

Synchrotron infrared measurements of carbonado have shown the presence of single nitrogen (type Ib) substitution and hydrogen (Garai et al., 2006), in contrast to aggregated N typical of conventional type Ia diamonds that have undergone prolonged high P-T annealing in the mantle.

Carbonado has been dated by Ozima and Tatsumoto (1997) and Sano et al. (2002), on samples derived from conglomerates (again, see figure 2) that have been reworked over a period from at least 1.7 Ga to approximately 3.8 Ga (Pedreira and De Waele, 2008). It is relevant to note that, unlike the dating of conventional diamond, which is based on trapped mineral inclusions (garnet, pyroxene, and sulfides), the age of carbonado discussed in this review was determined directly on diamond. Following a robust chemical protocol of acid dissolution to remove all nondiamond material, the cleansed carbonado was subjected to two different instrumental methods of analyses. Ozima and Tatsumoto (1997) used high-resolution mass spectrometry on carat-sized samples from the Central African Republic, while Sano et al. (2002) employed an ion probe that allowed for micron-sized spot analyses on larger samples from Brazil. Both studies report ages of 2.6–3.8 Ga on implanted radiogenic lead. Although this method of age determination is unconventional, it is important to note that the Archean result is consistent with trapped crustal inclusions (Sano et al., 2002) of zircon (1.7–3.6 Ga), rutile (3.9 Ga), and quartz (3.2 Ga), and with the antiquity of the basement in the São Francisco craton, which is 3.3–3.7 Ga (Barbosa and Sabate, 2004).

In summary, the chemical and physical characteristics of carbonado point to marked similarities with rapidly quenched type Ib diamonds and CVD diamond, both of which contain significant hydrogen. But there are also major differences: carbonado has pores and patinas with distinctions in C and N isotopes, an absence of mantle minerals, and the presence of exotic metal inclusions. Carbonado is unquestionably one of the most unusual forms of diamond ever reported. Because it has never been found in typical diamond-bearing rocks, the many proposed origins are varied, and none are uniformly accepted.

PROPOSED ORIGINS

Theories on the genesis of carbonado fall into five categories:

  1. Meteoritic impact (Smith and Dawson, 1985)
  2. Growth and sintering in the crust or mantle (Burgess et al., 1998; Ishibashi et al., 2012; Chen and Van Tendeloo, 1999; Heaney et al., 2005; Kagi and Fukura, 2008; Ketcham and Koeberl, 2013)
  3. Subduction (De Carli, 1997; Irifune et al., 2004)
  4. Radioactive ion implantation of carbon substrates (Kaminsky, 1991; Ozima et al., 1991; Shibata et al., 1993; Kagi et al., 1994; Daulton and Ozima, 1996; Ozima and Tatsumoto, 1997)
  5. Extraterrestrial (Haggerty, 1996, 2014)

Meteoritic Impact. This model was based on a correlation with the Bangui magnetic anomaly in the Central African Republic. Originally thought to be a buried iron meteorite, it was subsequently shown to be a crustal-derived banded iron ore body (Regan and Marsh, 1982), similar to the magnetic anomaly and giant iron ore deposit in Kursk, Russia (Taylor et al., 2014). Because the C-isotopic composition of carbonado is very light (δ13C = –21 to –34‰), the presence of biologically derived organic material in the target rocks is assumed. The impact model is unlikely because the C substrate, necessarily of cyanobacteria at ~3.8 Ga, would have been inordinately large (estimated at several cubic km and uncontaminated by crustal material), to account for the estimated two metric tons of carbonado recovered to date (Haggerty, 2014). In addition, the known occurrences of meteorite-impact diamonds (Arizona, United States; Ries, Germany; and Popigai, Russia) are discrete microdiamonds rather than carbonado (Frondel and Marvin, 1967; Hough et al., 1995; Shelkov et al., 1997).

Growth and Sintering in the Crust or Mantle. Some models propose catalytically assisted C-saturated “fluids” in the crust or the mantle. Such fluids provide a source of carbon and a medium capable of drastically decreasing the P-T stability limits of diamond from the traditional 5–6 GPa and 1200–1300°C, at a depth of 200 km or more (Shirey and Shigley, 2013). These “fluids” are hydrous, supercritical (i.e., beyond the point of coexisting fluid + vapor), and intensely oxidized so that diamond crystallization is unlikely, and diamond survival even less so. An analogy with loosely aggregated framesite, found in mantle-derived kimberlites, has also been suggested, but is unsatisfactory because the diamonds are semiprecious, free of pores and patina, and lack the highly reduced mineral suite of metals, carbides, and nitrides.

Subduction. Although carbonado is present in meta-conglomerates (again, see figure 2), these robustly cemented diamonds are very different from the ultra-high-pressure, subducted, metamorphic diamonds found in continental collision zones in Norway, China, Kazakhstan, Greece, and Germany (Ogasawara, 2005). The diamonds at these localities are single crystals and are armored by zircon, garnet, pyroxene, and amphibole that acted as insulating capsules. Sintering would be necessary to form carbonado. This is possible at high pressures and temperatures in the mantle, but the process would have incorporated one or more mantle minerals such as olivine, garnet, pyroxene, and spinel, none of which are observed. Moreover, the inferred subducted plates are oceanic and basaltic in composition and on transformation at high P-T would produce large concentrations of garnet + pyroxene (namely eclogite), which again is not encountered. Transport to Earth’s surface is either not considered or is tentatively ascribed to deep mantle volcanic plumes in both the subduction and radiation models (below).

Radioactive Ion Implantation of Carbon Substrates. Radiation-induced diamond is on the scale of nanometers and cannot account for larger diamonds in the micrometer to millimeter size range found in carbonado. Once diamond is formed, low-energy implantation alters the atomic structure and turns the diamond green; high-energy ion doses produce graphite rather than additional diamond (Kalish and Prawer, 1995). There were no coal deposits at 2–3 Ga, and the radiation-induced diamonds recovered from very rare carburanium (U-rich hydrocarbon) are low in abundance and nanometer in size. Proposals of radiation sintering, and even pore formation, are equally untenable.

Extraterrestrial Origin. The extraterrestrial (ET) model was initially proposed because traditional earthbound scenarios failed to account for major characteristics of carbonado, namely diamond porosity, patina, polycrystallinity, rarity, and location (Haggerty, 2014). Pores are incompatible with high-pressure environments; therefore, carbonado cannot have formed under the same conditions in which conventional diamonds form in the mantle at depths of approximately 200 km. The pores in carbonado (again, see figures 3 and 4) are similar to vesicles in basalts that degassed at low pressures under near-surface conditions from a molten or semi-molten magma. This rules out an origin for carbonado in the crust or the mantle, because liquefaction of carbon is not readily accomplished. In fact, diamond is solid in Earth’s core (6,380 km and approximately 350 GPa and 7000 K; Bundy et al., 1996; Oganov et al., 2013). Consequently, none of the interpreted melt-like features in carbonado (figures 5–7) can possibly be of terrestrial origin. Furthermore, not a single carbonado has been reported from kimberlite-lamproite suites in the nearly 700 metric tons of diamond mined since about 1900 (Levinson et al., 1992). As noted above, carbonado differs from conventional diamond in several respects:

  1. Hydrogen is prominent and N is dispersed, which is the case for <1% of conventional diamonds (i.e., type Ib).
  2. Combined N and C isotopes are distinctly not terrestrial (figure 10).
  3. There are remarkable similarities to diamonds formed by carbon vapor deposition (figure 11), a process that requires vacuum conditions and plasma temperatures that cannot possibly be accomplished in any natural environment on Earth.
  4. Carbonado lacks the characteristic suite of diamond inclusion minerals such as Cr-garnet, Na-Al-pyroxene, Mg-olivine, Mg-chromite, and Fe-Ni-sulfides, and is instead replaced by exotic, reduced metal alloys and minerals.

The ET scenario posits that carbonado originated from carbon-rich, diamond-bearing stellar bodies and/or disrupted C-bearing planets (Haggerty, 2014). All of the characteristic features of carbonado are satisfied: CVD diamond is the sintering glue to microdiamonds in carbonado; the loss of interstellar H produced the pores, and the patina and flow textures are stellar or interstellar high-vacuum melt products. The model further proposes that carbonado was transported to Earth as a large diamond meteorite or as smaller diamond “plums” in a carbonaceous meteoritic matrix, possibly during the Late Heavy Bombardment (3.8–4.2 Ga), in which the inner solar system was pummeled by meteorites (Fassett and Minton, 2013; Abramov et al., 2013). The numerous craters on the moon are considered evidence of the bombardment (Marchi et al., 2013). The theoretical age of this event corresponds to the oldest age determined for carbonado (3.8 ± 1.8 Ga). This would account for its rarity as a single known occurrence on the São Francisco and Congo cratons, which were once joined geologically as the supercontinents of Nuna and Rodinia. Carbonado was undoubtedly widespread during the bombardment, but the carbonado falls were largely into the expansive oceans that existed at that time. Supercontinent disruption and subduction followed, leaving only the preserved remnants of carbonado on an island that is today split between Brazil and the Central African Republic.

The recent discovery of patches of sub-micron diamonds in Libyan desert glass, a high-silica natural glass that is thought to be of cometary origin (Kramers et al., 2013), lends credence to the ET model for carbonado. This view is supported by the growing lines of evidence for (1) synthetically produced diamond-like glass (Shumilova et al., 2016a, b); (2) nanodiamond encased in glassy carbon shells in the interstellar media (Yastrebov and Smith, 2009); and (3) glassy carbon and nanodiamond produced experimentally (Shiell et al., 2016) and in supernova shock waves (Stroud et al., 2011). Another supporting fact is the discovery of asteroid 2008 TC3, which was tracked upon entering Earth’s atmosphere and landed in North Sudan as a fragmented, diamond-bearing ureilite (Miyahara et al., 2015). Unusual in several respects, the meteorite contains diamonds measuring approximately 100 µm. These are exceptionally large for ureilites, whose diamonds typically measure 1–5 µm, and substantially larger than nanodiamonds of pre-solar origin in carbonaceous chondrites. These reports are complemented by the unexpected discovery that Mercury has a crust of graphite, now covered by volcanic rocks but exposed in meteorite craters (Peplowski et al., 2016), that may prove to be diamond bearing.

CONCLUSIONS

Carbonado (figure 12) is the most unusual form of diamond on Earth. Despite many mineralogical clues not observed in conventional diamonds, its mode of origin remains largely unexplained. Discovering the origin of carbonado would herald a whole new mode of diamond formation and could represent a remarkable form of extraterrestrial carbon delivery to Earth. The extraterrestrial model, although conceptual and supported by astrophysical data, will only be vindicated by the discovery of carbonado in the asteroid belt by remote sensing, or by an observed diamond meteorite fall that is dark in color, porous, and patinaed.

Diamond octahedron, diamond bort, and carbonado
Figure 12. The origin of carbonado diamond (far right) has yet to be definitively established. Uncovering their formation would represent a scientific breakthrough. Left to right: The 9.49 ct yellow diamond octahedron is a gift of the Oppenheimer Student Collection. The 109.47 ct diamond bort is a gift of Richard Vainer. The 118.01 ct carbonado, a gift of Stephen Haggerty, is from the Central African Republic. GIA Collection nos. 11953, 31602, and 40108. Photo by Robert Weldon/GIA.

Prof. Haggerty is distinguished research professor in the Department of Earth and Environment at Florida International University in Miami.

Fieldwork for this study was supported by a faculty research grant from the University of Massachusetts Amherst, and by De Beers. Laboratory work was supported by the National Science Foundation and Florida International University. Thanks to Jose Ricardo Pisani and the late Jeff Watkins, who provided enormous logistical help and hospitality during fieldwork in Brazil. Thanks also to my many colleagues and critics, from whom I’ve benefited enormously in active discussions on the controversial issues surrounding the origin of carbonado. And lastly to the reviewers for detailed and constructive comments that led to improvements in presentation. To all I express my sincere appreciation.

Abramov O., Kring D.A., Mojzsis S.J. (2013) The impact environment of the Hadean Earth. Chemie der Erde - Geochemistry, Vol. 73, No. 3, pp. 227–248, http://dx.doi.org/10.1016/j.chemer.2013.08.004

Barbosa J.S.F., Sabate P. (2004) Archean and Paleoproterozoic crust of the São Francisco craton, Bahia, Brazil: Geodynamic features. Precambrian Research, Vol. 133, No. 1-2, pp. 1–27, http://dx.doi.org/10.1016/j.precamres.2004.03.001

Bundy F.P., Bassett W.A., Weathers M.S., Hemley R.J., Mao H.U., Goncharov A.F. (1996) The pressure-temperature phase and transformation diagram for carbon; updated through 1994. Carbon, Vol. 34, No. 2, pp. 141–153, http://dx.doi.org/10.1016/0008-6223(96)00170-4

Burgess R., Johnson L.H., Mattey D.P., Harris J.W., Turner G. (1998) He, Ar and C isotopes in coated and polycrystalline diamonds. Chemical Geology, Vol. 146, No. 3-4, pp. 205–217, http://dx.doi.org/10.1016/S0009-2541(98)00011-4

Burns R.C., Davies G.J. (1992) Growth of synthetic diamond. In J.E. Field, Ed., The Properties of Natural and Synthetic Diamond. Academic Press, London, pp. 395–422.

Cartigny P., Harris J.W., Javoy M. (1998) Eclogitic diamond formation at Jwaneng: No room for a recycled component. Science, Vol. 280, No. 5368, pp. 1421–1424, http://dx.doi.org/10.1126/science.280.5368.1421

Chen J.H., Van Tendeloo G. (1999) Microstructure of tough polycrystalline natural diamond. Journal of Electron Microscopy, Vol. 48, No. 2, pp. 121–129, http://dx.doi.org/10.1093/oxfordjournals.jmicro.a023658

Clark C.D., Collins A.T., Woods G.S. (1992) Absorption and luminescence spectroscopy. In J.E. Field, Ed., The Properties of Natural and Synthetic Diamond. Academic Press, London, pp. 35–79.

Daulton T.L., Ozima M. (1996) Radiation-induced diamond formation in uranium-rich carbonaceous materials. Science, Vol. 271, No. 5253, pp. 1260–1263, http://dx.doi.org/10.1126/science.271.5253.1260

De Carli P.S. (1997) Carbonado origin: Impact vs. subduction. Abstract, American Geophysical Union Meeting, Baltimore, Maryland, S333.

De S., Heaney P.J., Hargraves R.B., Vicenzi E.P., Taylor P.T. (1998) Microstructural observations of polycrystalline diamond: a contribution to the carbonado conundrum. Earth and Planetary Science Letters, Vol. 164, No. 3-4, pp. 421–433, http://dx.doi.org/10.1016/S0012-821X(98)00229-5

De S., Heaney P.J., Vicenzi E.P., Wang J.H. (2001) Chemical heterogeneity in carbonado, an enigmatic polycrystalline diamond. Earth and Planetary Science Letters, Vol. 185, No. 3-4, pp. 315–330, http://dx.doi.org/10.1016/S0012-821X(00)00369-1

De Waele B., Johnson S.P., Pisarevsky S.A. (2008) Palaeoproterozoic to Neoproterozoic growth and evolution of the eastern Congo Craton: Its role in the Rodinia puzzle. Precambrian Research, Vol. 160, No. 1-2, pp. 127–141, http://doi.org/10.1016/j.precamres.2007.04.020

Dominguez J.M.L. (1996) As coberturas plataformais do proterzoico medio e superior (The Middle and Upper Proterozoic). In Mapa Geologico Do Estado Da Bahia. Secretaria da Geologia E Recursos Minerais, Salvador, pp. 105–125.

Fassett C.I., Minton D.A. (2013) Impact bombardment of the terrestrial planets and the early history of the solar system. Nature Geoscience, Vol. 6, No. 7, pp. 520–524, http://dx.doi.org/10.1038/ngeo1841

Frondel C., Marvin U.B. (1967) Lonsdaleite, a hexagonal polymorph of diamond. Nature, Vol. 214, No. 5088, pp. 587–589, http://dx.doi.org/10.1038/214587a0

Garai J., Haggerty S.E., Rekhi S., Chance M. (2006) Infrared absorption investigations confirm the extraterrestrial origin of carbonado diamonds. The Astrophysical Journal Letters, Vol. 653, No. 2, pp. L153–L156, http://dx.doi.org/10.1086/510451

Gorshkov A.I., Titkov S.V., Pleshakov A.M., Sivtov A.V., Bershov L.V. (1996) Inclusions of native metals and other mineral phases into carbonado from Ubangi region (Central Africa). Geology of Ore Deposits, Vol. 38, pp. 114–119.

Haggerty S.E. (1996) Diamond-carbonado: Models for a new meteorite class of circum-stellar or solar system origin. Abstract, American Geophysical Union, Spring meeting, Baltimore, p. S143.

Haggerty S.E. (2014) Carbonado: Physical and chemical properties, a critical evaluation of proposed origins, and a revised genetic model. Earth-Science Reviews, Vol. 130, pp. 49–72, http://dx.doi.org/10.1016/j.earscirev.2013.12.008

Heaney P.J., Vicenzi E.P., De S. (2005) Strange diamonds: The mysterious origins of carbonado and framesite. Elements, Vol. 1, No. 2, pp. 85–89, http://dx.doi.org/10.2113/gselements.1.2.85

de Heer W.A., Poncharal P., Berger C., Gezo J., Song Z., Bettini J., Ugarte D. (2005) Liquid carbon, carbon-glass beads, and the crystallization of carbon nanotubes. Science, Vol. 307, No. 5711, pp. 907–910, http://dx.doi.org/10.1126/science.1107035

Hough R.M., Gilmour I., Pillinger C.T., Arden J.W., Gilkes K.W.R., Yuan J., Milledge H.J. (1995) Diamond and silicon carbide in impact melt rock from the Ries impact crater. Nature, Vol. 378, No. 6552, pp. 41–44, http://dx.doi.org/10.1038/378041a0

Irifune T., Kurio A., Sakamoto S., Inoue T., Sumiya H., Funakoshi K. (2004) Formation of pure polycrystalline diamond by direct conversion of graphite at high pressure and high temperature. Physics of the Earth and Planetary Interiors, Vol. 143-144, pp. 593–600, http://dx.doi.org/10.1016/j.pepi.2003.06.004

Ishibashi H., Kagi H., Sakuai H., Ohfuji H., Sumino H. (2012) Hydrous fluid as the growth media of natural polycrystalline diamond, carbonado: Implication from IR spectra and microtextural observations. American Mineralogist, Vol. 97, No. 8-9, pp. 1366–1372, http://dx.doi.org/10.2138/am.2012.4097

Jones A.P., Beard A., Milledge H.J., Cressey G., Kirk C., DeCarli P. (2003) A new nitride mineral in carbonado. Eighth International Kimberlite Conference Abstracts, Victoria, Canada, p. A95.

Kagi H., Fukura S. (2008) Infrared and Raman spectroscopic observations of Central African carbonado and implications for its origin. European Journal of Mineralogy, Vol. 20, No. 3, pp. 387–393, http://dx.doi.org/10.1127/0935-1221/2008/0020-1817

Kagi H., Takahashi K., Hidaka H., Masuda A. (1994) Chemical properties of Central African carbonado and its genetic implications. Geochimica et Cosmochimica Acta, Vol. 58, No. 12, pp. 2629–2638, http://dx.doi.org/10.1016/0016-7037(94)90133-3

Kalish R., Prawer S. (1995) Graphitization of diamond by ion impact: Fundamentals and applications. Nuclear Instruments and Methods in Physics Research Section B, Vol. 106, No. 1-4, pp. 492–499, http://dx.doi.org/10.1016/0168-583X(95)00758-X

Kaminsky F.V. (1991) Carbonado and yakutite: Properties and possible genesis. In H.O.A. Meyer and O.H. Leonardos, Eds., Proceedings of the 5th International Kimberlite Conference, Araxá, Brazil, CPRM Special Publication, pp. 136–143.

Ketcham R.A., Koeberl C. (2013) New textural evidence on the origin of carbonado diamond: An example of 3-D petrography using X-ray computed tomography. Geosphere, Vol. 9, No. 5, pp. 1336–1347, http://dx.doi.org/10.1130/GES00908.1

Kramers J.D., Andreoli M.A.G., Atanasova M., Belyanin G.A., Blocke D.L., Franklyn C., Harris C., Lekgoathi M., Montross C.S., Ntsoane T., Pischedda V., Segonyane P., Viljoen K.S., Westraadt J.E. (2013) Unique chemistry of a diamond-bearing pebble from the Libyan Desert Glass strewnfield, SW Egypt: Evidence for a shocked comet fragment. Earth and Planetary Science Letters, Vol. 382, pp. 21–31, http://dx.doi.org/10.1016/j.epsl.2013.09.003

Leonardos O.H. (1937) Diamante e Carbonado no Estado da Bahia. Metallurgia Servisio De Fomento da Produccao Mineral, Vol. 19, pp. 1–23.

Levinson A.A., Gurney J.J., Kirkley M.B. (1992) Diamond sources and production: Past, present, and future. G&G, Vol. 28, No. 4, pp. 234–254, http://dx.doi.org/10.5741/GEMS.28.4.234

Magee C.W., Taylor W.R. (1999) Constraints from luminescence on the history and origin of carbonado. In J.J. Gurney et al., Eds., Proceedings of the VIIth International Kimberlite Conference, Red Roof Design Publications, Cape Town, pp. 529–532.

Makeev A.B., Ivanuch W., Obyden S.K., Saparin G.V., Filippov V.N. (2002) Mineralogy, composition of inclusions, and cathodoluminescence of carbonado from Bahia State, Brazil. Geology of Ore Deposits, Vol. 44, No. 2, pp. 87–102.

Marchi S., Bottke W.F., Cohen B.A., Wünnemann K., Kring D.A., McSween H.Y., De Sanctis M.C., O’Brien D.P., Schenk P., Raymond C.A., Russell C.T. (2013) High-velocity collisions from the lunar cataclysm recorded in asteroidal meteorites. Nature Geoscience, Vol. 6, No. 4, pp. 303–307, http://dx.doi.org/10.1038/ngeo1769

Mitchell T.M., Toy V., Di Toro G., Renner J., Sibson R.H. (2016) Fault welding by pseudotachylyte formation. Geology, Vol. 44, No. 12, pp. 1059–1062, http://dx.doi.org/10.1130/G38373.1

Miyahara M., Ohtani E., El Goresy A., Lin Y., Feng L., Zhang J.-C., Gillet P., Nagase T., Muto J., Nishijima M. (2015) Unique large diamonds in an ureilite from Almahata Sitta 2008 TC3 asteroid. Geochimica et Cosmochimica Acta, Vol. 163, pp. 14–26, http://dx.doi.org/10.1016/j.gca.2015.04.035

Nadolinny V.A., Shatsky V.S., Sobolev N.V., Twitchen D.J., Yuryeva O.P., Vasilevsky I.A., Lebedev V.N. (2003) Observation and interpretation of paramagnetic defects in Brazilian and Central African carbonados. American Mineralogist, Vol. 88, No. 1, pp. 11–17, http://dx.doi.org/10.2138/am-2003-0102

Oganov A.R., Hemley R.J., Hazen R.M., Jones A.P. (2013) Structure, bonding, and mineralogy of carbon at extreme conditions. In R.M Hazen, A.P. Jones., and J.A. Baross, Eds., Carbon in Earth. Reviews in Mineralogy & Geochemistry, Vol. 75. Mineralogical Society of America, Chantilly, VA, pp. 47–77.

Ogasawara Y. (2005) Microdiamonds in ultrahigh-pressure metamorphic rocks. Elements, Vol. 1, No. 2, pp. 91–96, http://dx.doi.org/10.2113/gselements.1.2.91

Orlov Y.L. (1977) The Mineralogy of the Diamond. John Wiley & Sons, New York, 235 pp.

Ozima M., Tatsumoto M. (1997) Radiation-induced diamond crystallization: Origin of carbonados and its implications on meteorite nano-diamonds. Geochimica et Cosmochimica Acta, Vol. 61, No. 2, pp. 369–376, http://dx.doi.org/10.1016/S0016-7037(96)00346-8 Ozima M., Zashu S., Tomura K., Matsuhisa Y. (1991) Constraints from noble-gas contents on the origin of carbonado diamonds. Nature, Vol. 351, No. 6326, pp. 472–474, http://dx.doi.org/10.1038/351472a0

Pedreira A.J., De Waele B. (2008) Contemporaneous evolution of the Paleoproterozoic-Mesoproterozoic sedimentary basins of the São Francisco–Congo craton. In R.J. Pankhurst, R.A.J. Trouw, B.B. de Brito Neves, and M.J. De Wit, Eds., West Gondwana: Pre-Cenozoic Correlations across the South Atlantic Region. Geological Society of London Special Publication 294, pp. 33–48.

Peplowski P.N., Klima R.L., Lawrence D.J., Ernst C.M., Denevi B.W., Frank E.A., Goldsten J.O., Murchie S.L., Nittler L.R., Solomon S.C. (2016) Remote sensing evidence for an ancient carbon-bearing crust on Mercury. Nature Geoscience, Vol. 9, No. 4, pp. 273–276, http://dx.doi.org/10.1038/ngeo2669

Petrovsky V.A., Shiryaev A.A., Lyutoev V.P., Sukharev A.E., Martins M. (2010) Morphology and defects of diamond grains in carbonado: Clues to carbonado genesis. European Journal of Mineralogy, Vol. 22, No. 1, pp. 35–47, http://dx.doi.org/10.1127/0935-1221/2010/0022-1978

Regan R.D., Marsh B.D. (1982) The Bangui magnetic anomaly: Its geological origin. Journal of Geophysical Research: Solid Earth. Vol. 87, No. B2, pp. 1107–1120, http://dx.doi.org/10.1029/JB087iB02p01107

Rondeau B., Sautter V., Barjon J. (2008) New columnar texture of carbonado: Cathodoluminescence study. Diamond and Related Materials, Vol. 17, No. 11, pp. 1897–1901, http://dx.doi.org/10.1016/j.diamond.2008.04.006

Sano Y., Yokochi R., Terada K., Chaves M.L., Ozima M. (2002) Ion microprobe Pb–Pb dating of carbonado, polycrystalline diamond. Precambrian Research, Vol. 113, No. 1-2, pp. 155–168, http://dx.doi.org/10.1016/S0301-9268(01)00208-X

Sato Y., Kamo M. (1992) Synthesis of diamond from the vapor phase. In J.E. Field, Ed., The Properties of Natural and Synthetic Diamond, pp. 423–469. Academic Press, London.

Shelkov D., Verkhovsky A.B., Milledge H.J., Pillenger C.T. (1997) Carbonado: A comparison between Brazilian and Ubangui sources with other forms of microcrystalline diamond based on carbon and nitrogen isotopes. Russian Geology and Geophysics, Vol. 38, No. 2, pp. 315–322.

Shibata K., Kamioka H., Kaminsky F.V., Koptil V.I., Svisero D.P. (1993) Rare earth element patterns of carbonado and yakutite: Evidence for their crustal origin. Mineralogical Magazine, Vol. 57, No. 389, pp. 607–611, http://dx.doi.org/10.1180/minmag.1993.057.389.05

Shiell T.B., McCulloch D.G., Bradby J.E., Haberl B., Boehler R., McKenzie D.R. (2016) Nanocrystalline hexagonal diamond formed from glassy carbon. Scientific Reports, Vol. 6, No. 1, article no. 37232, http://dx.doi.org/10.1038/srep37232

Shirey S.B., Shigley J.E. (2013) Recent advances in understanding the geology of diamonds. G&G, Vol. 49, No. 4, pp. 188–222, http://dx.doi.org/10.5741/GEMS.49.4.188

Shumilova T.G., Isaenko S.I., Tkachev S.N. (2016a) Diamond formation through metastable liquid carbon. Diamond and Related Materials, Vol. 62, pp. 42–48, http://dx.doi.org/10.1016/j.diamond.2015.12.015

Shumilova T.G., Tkachev S.N., Isaenko S.I., Shevchuk S.S., Rappenglück M.A., Kazakov V.A. (2016b) A “diamond-like star” in the lab. Diamond-like glass. Carbon, Vol. 100, pp. 703–709, http://dx.doi.org/10.1016/j.carbon.2016.01.068

Smith J.V., Dawson J.B. (1985) Carbonado: Diamond aggregates from early impacts of crustal rocks? Geology, Vol. 13, No. 5, pp. 342–343, http://dx.doi.org/10.1130/0091-7613(1985)13%3C342:CDAFEI%3E2.0.CO;2

Stroud R.M., Chisholm M.F., Heck P.R., Alexander C.M.O’D., Nittler L.R. (2011) Supernova shock-wave-induced co-formation of glassy carbon and nanodiamond. The Astrophysical Journal Letters, Vol. 738, No. 2, pp. L27–L32, http://dx.doi.org/10.1088/2041-8205/738/2/L27

Svisero D.P. (1995) Distribution and origin of diamonds in Brazil: An overview. Journal of Geodynamics, Vol. 20, No. 4, pp. 493–514, http://dx.doi.org/10.1016/0264-3707(95)00017-4

Taylor P.T., Kis K.I., Wittmann G. (2014) Satellite-altitude horizontal magnetic gradient anomalies used to define the Kursk magnetic anomaly. Journal of Applied Geophysics, Vol. 109, pp. 133–139, http://dx.doi.org/10.1016/j.jappgeo.2014.07.018

Torsvik T.H. (2003) The Rodinia jigsaw puzzle. Science, Vol. 300, No. 5624, pp. 1379–1381, http://dx.doi.org/10.1126/science.1083469

Trueb L.F., Butterman W.C. (1969) Carbonado: A microstructural study. American Mineralogist, Vol. 54, No. 3-4, pp. 412–425.

Trueb L.F., De Wys E.C. (1969) Carbonado: Natural polycrystalline diamond. Science, Vol. 165, No. 3895, pp. 799–802, http://dx.doi.org/10.1126/science.165.3895.799

Vicenzi E.P., Heaney P.J. (2001) The carbon and nitrogen isotopic composition of carbonado diamond: an in-situ study. Eleventh Annual VM Goldschmidt Conference, A513.

Wang W., Lu R., Moses T. (2009) Photoluminescence features of carbonado diamonds. GIA News from Research, July 21, https://www.gia.edu/gia-news-research-nr72109

Yastrebov S., Smith R. (2009) Nanodiamonds enveloped in glassy carbon shells and the origin of the 2175 Å optical extinction feature. The Astrophysical Journal, Vol. 697, No. 2, pp. 1822–1826, http://dx.doi.org/10.1088/0004-637X/697/2/1822

Yokochi R., Ohnenstetter D., Sano Y. (2008) Intragrain variation in δ13C and nitrogen concentration associated with textural heterogeneities of carbonado. Canadian Mineralogist, Vol. 46, No. 5, pp. 1283–1296, http://dx.doi.org/10.3749/canmin.46.5.1283