168 CARBONADO DIAMOND GEMS & GEMOLOGY SUMMER 2017
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, respec-
tively, previously shared a common geological set-
ting 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 dia-
mond 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 tech-
niques, 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, superfi-
cial appearance, and physical and chemical proper-
ties (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
A REVIEW OF PROPERTIES AND ORIGIN
Stephen E. Haggerty
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 dia-
mond 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.
• Carbonado, found only in Brazil and the Central
African Republic, is distinguished from conventional
diamond by its pores, patina, surfaces, and polycrys-
• Although carbonado has been known since 1841, its
origin or host rock has yet to be identified.
• The extraterrestrial model of carbonado origin, one of
five theories, posits that it formed from carbon-rich,
diamond-bearing stellar bodies that were transported
to Earth by meteorite about four billion years ago.
See end of article for About the Author and Acknowledgments.
GEMS & GEMOLOGY, Vol. 53, No. 2, pp. 168–179,
© 2017 Gemological Institute of America
CHARACTERISTICS OF CARBONADO
Carbonado is typically found in five major size cate-
gories: >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 typ-
ically equidimensional (in millimeter to centimeter
CARBONADO DIAMOND GEMS & GEMOLOGY SUMMER 2017 169
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 is-
land. Right: Separation of the microcontinent into two cratonic blocks, now Brazil and the Central African Repub-
lic, took place during the breakup of Gondwanaland about 180 million years ago. 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).
1.1 Ga belts
Figure 2. A: Scene from
the carbonado site in
Bahia, Brazil. B: Boulder
of Tombodor conglomer-
ate, the carbonado host
rock. C: Polished con-
glomerate surface in a
streambed. D: Artisanal
mining of Brazilian allu-
vial carbonado. Photos
by Robert Weldon (A
and C) and Stephen E.
Haggerty (B and D).
sizes), although some are elongated (figure 3); they
are seldom rounded.
Carbonado is opaque, composed of randomly ori-
ented diamond crystallites that impede light refrac-
tion and increase absorption. Color varies from black
and putty gray to shades of brown (figure 3), deep pur-
ple 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 poly-
crystallinity (figures 8 and 9) distinguish carbonado
from conventional diamonds.
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 micro diamond cutting points in-
creases with porosity. This was a sought-after prop-
erty 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/cm3and
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 Koe-
berl, 2013), but the material’s permeability is very
low because the pores are free of infiltrating hydro -
thermal precipitates that abound in surface pores
(again, see figure 4). The spherical pores in car-
bonado are unlike those in other polycrystalline di-
amond such as framesite, where the open spaces are
at adjoining crystal faces and the shapes are irregu-
larly polyhedral. In other polycrystalline diamonds,
the open spaces are microns in width and either ra-
dial (in non-gem-quality ballas) or parallel (in fibrous
170 CARBONADO DIAMOND GEMS & GEMOLOGY SUMMER 2017
Figure 3. The carbonado 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.
Figure 4. Open pores in
carbonado (first five
photos) and pores cov-
ered by a surface
patina of nanodiamond
approximately 5 μm
thick (lower right).
Photos by Stephen E.
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 vol-
canic 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 orna-
mentation, 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 dia-
mond. On the other hand, the patina itself may rep-
resent 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
CARBONADO DIAMOND GEMS & GEMOLOGY SUMMER 2017 171
Figure 5. Typical melt-
like patinas and flow
ornamentation on car-
bonado. Photos by
Stephen E. Haggerty.
Figure 6. Melt marbles
and flow patterns on
carbonado. Photos by
Stephen E. Haggerty.
diamond surfaces, less so to the ridges and mounds.
The pore-rich surfaces are distinctly sluggish and er-
ratic in response, possibly due to crystal discontinu-
ities 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. Poly-
crystalline cubes measuring 5 to approximately 20
µm are common. Encased in very fine diamond (<1–
5 μm), the matrix is tightly fused with angular inter-
stices and rounded pores (figure 8). Scanning electron
microscopy (SEM) images illustrate the distribution
of diamond cleavage surfaces, hopper crystals, skele-
tal 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 Petro-
vsky 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
 that follow the fluorite twin law (figure 8). The
solid cubes are colorless and, although fine grained,
appear to be translucent. Diamond cubes and cuboc-
tahedra are routinely synthesized in metallic cata-
lysts 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 car-
bonado grains are similar to conventional diamonds.
Hardness is also similar, but there are data indicating
172 CARBONADO DIAMOND GEMS & GEMOLOGY SUMMER 2017
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.
Figure 7. Slickenside
patterns and melting of
with later pits and cra-
tering. The melt layer is
about 20 μm thick.
Photos by Stephen E.
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 car-
bonado 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 pri-
mary 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 re-
ducing 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 oxi-
dized 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). Ni-
trogen 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 dis-
tribution of C and N shows that the compositional
field for carbonado is distinctly different from that of
conventional diamonds (figure 10).
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 at-
tributed 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.
Cathodoluminescence of large (approximately 200
μm) monocrystals of diamond in carbonado exhibit or-
ange and green tones (Magee and Taylor, 1999; De et
al., 2001; Yokochi et al., 2008). However, blue lumi-
nescence 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 con-
trast 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 Tat-
sumoto (1997) and Sano et al. (2002), on samples de-
rived from conglomerates (again, see figure 2) that
CARBONADO DIAMOND GEMS & GEMOLOGY SUMMER 2017 173
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.
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 deter-
mined directly on diamond. Following a robust chem-
ical 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-res-
olution 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 radi-
ogenic lead. Although this method of age determina-
tion is unconventional, it is important to note that the
Archean result is consistent with trapped crustal in-
clusions (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 character-
istics of carbonado point to marked similarities with
rapidly quenched type Ib diamonds and CVD dia-
mond, both of which contain significant hydrogen.
But there are also major differences: carbonado has
pores and patinas with distinctions in C and N iso-
topes, an absence of mantle minerals, and the pres-
ence of exotic metal inclusions. Carbonado is
unquestionably one of the most unusual forms of di-
amond ever reported. Because it has never been found
in typical diamond-bearing rocks, the many proposed
origins are varied, and none are uniformly accepted.
Theories on the genesis of carbonado fall into five
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,
3. Subduction (De Carli, 1997; Irifune et al., 2004)
4. Radioactive ion implantation of carbon sub-
strates (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)
174 CARBONADO DIAMOND GEMS & GEMOLOGY SUMMER 2017
Figure 10. A paired stable
isotope plot of C vs. N for
conventional diamond (top)
and carbonado (bottom).
The compositional separa-
tion shows that carbonado
and deep Earth diamonds
are unrelated. Open sym-
bols are for eclogitic dia-
monds from Kim berley,
South Africa; filled symbols
are for diamonds from Jwa-
neng, Bot swana; both show
extreme variations. The
fields for peridotitic (typical
inclu sions are olivine,
clinopyroxene, and orthopy-
roxene), eclogitic (garnet
and clinopyroxene), and fi-
brous diamonds are from a
global database. Data for
conventional diamonds are
from Cartigny et al. (1998).
(Gt + Cpx)
(Ol + Cpx + Opx)
“low δ13C” group
N = –3 to 12.8‰
C = –24 to –31‰
Meteoritic Impact. This model was based on a cor-
relation 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 car-
bonado is very light (δ13C = –21 to –34‰), the pres-
ence of biologically derived organic material in the
target rocks is assumed. The impact model is un-
likely because the C substrate, necessarily of
cyanobacteria at ~3.8 Ga, would have been inordi-
nately large (estimated at several cubic km and un-
contaminated by crustal material), to account for the
estimated two metric tons of carbonado recovered to
date (Haggerty, 2014). In addition, the known occur-
rences 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 pro-
vide a source of carbon and a medium capable of dras-
tically 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 unsatis-
factory because the diamonds are semiprecious, free
of pores and patina, and lack the highly reduced min-
eral suite of metals, carbides, and nitrides.
Subduction. Although carbonado is present in meta-
conglomerates (again, see figure 2), these robustly ce-
mented diamonds are very different from the
ultra-high-pressure, subducted, metamorphic dia-
monds found in continental collision zones in Nor-
way, 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 car-
bonado. This is possible at high pressures and tem-
peratures 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 trans-
formation at high P-T would produce large concen-
trations of garnet + pyroxene (namely eclogite),
which again is not encountered. Transport to Earth’s
surface is either not considered or is tentatively as-
cribed 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 nanome-
ters and cannot account for larger diamonds in the mi-
crometer to millimeter size range found in carbonado.
Once diamond is formed, low-energy implantation al-
ters 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 radia-
CARBONADO DIAMOND GEMS & GEMOLOGY SUMMER 2017 175
Figure 11. PL spectra illustrating the similarity be-
tween 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).
PHOTON ENERGY (eV)
2.156 eV 2.463 eV
tion-induced diamonds recovered from very rare car-
buranium (U-rich hydrocarbon) are low in abundance
and nanometer in size. Proposals of radiation sinter-
ing, 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 poros-
ity, 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 con-
ventional 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-sur-
face 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 fea-
tures in carbonado (figures 5–7) can possibly be of ter-
restrial 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 dia-
monds (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
4. Carbonado lacks the characteristic suite of di-
amond inclusion minerals such as Cr-garnet,
Na-Al-pyroxene, Mg-olivine, Mg-chromite, and
Fe-Ni-sulfides, and is instead replaced by ex-
otic, 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 sat-
isfied: CVD diamond is the sintering glue to micro-
diamonds 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 me-
teoritic matrix, possibly during the Late Heavy Bom-
bardment (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 deter-
mined for carbonado (3.8 ± 1.8 Ga). This would ac-
count 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 car-
bonado 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 di-
amonds 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 car-
bonado. 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 car-
bon and nanodiamond produced experimentally
(Shiell et al., 2016) and in supernova shock waves
(Stroud et al., 2011). Another supporting fact is the dis-
covery 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 approxi-
mately 100 μm. These are exceptionally large for ure-
ilites, 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 Mer-
cury 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.
176 CARBONADO DIAMOND GEMS & GEMOLOGY SUMMER 2017
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. Discov-
ering the origin of carbonado would herald a whole
new mode of diamond formation and could repre-
sent a remarkable form of extraterrestrial carbon de-
livery 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 ob-
served diamond meteorite fall that is dark in color,
porous, and patinaed.
CARBONADO DIAMOND GEMS & GEMOLOGY SUMMER 2017 177
ABOUT THE AUTHOR
Prof. Haggerty is distinguished research professor in the Depart-
ment of Earth and Environment at Florida International University
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 enor-
mous 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 re-
viewers for detailed and constructive comments that led to im-
provements in presentation. To all I express my sincere
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Figure 12. The origin of
carbonado diamond (far
right) has yet to be defin-
itively established. Un-
covering their formation
would represent a scien-
tific breakthrough. Left
to right: The 9.49 ct yel-
low diamond octahedron
is a gift of the Oppen-
heimer Student Collec-
tion. 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.
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