Most diamonds tell a familiar story. They crystallize under crushing pressure some two hundred kilometers down in Earth’s mantle and ride to the surface in kimberlite eruptions[1]. Carbonado refuses that story. It is a black, porous aggregate of countless small diamond crystals, and in the nearly seven hundred metric tons of diamond mined from kimberlite and lamproite since about 1900, not one carbonado has been reported[1]. Its only known sources are alluvial and placer deposits in Brazil and the Central African Republic[2,1].
That absence invited a radical proposal. In 2006 Jozsef Garai, Stephen Haggerty, Sandeep Rekhi and Mark Chance published synchrotron infrared measurements of carbonado made at Brookhaven National Laboratory[3,4]. They reported hydrogen and nitrogen present as isolated single atoms rather than the aggregated clusters typical of mantle diamonds that have been annealed for long periods at high pressure and temperature[3,1]. Comparing the spectra with laboratory-grown diamond made by chemical vapor deposition and with presolar diamonds extracted from meteorites, the authors concluded that carbonado formed in a hydrogen-rich interstellar setting[3].
The U.S. National Science Foundation, which funded the work, summarized the claim for the public in January 2007: the stones may have formed in carbon-rich debris from exploding stars and reached Earth inside bodies a kilometer or more across[5]. Haggerty’s later reviews widened the frame, proposing that carbonado originated in “carbon-rich, diamond-bearing stellar bodies and/or disrupted C-bearing planets”[1,6].
The case rests on several features that are difficult to reconcile with the mantle. Carbonado’s carbon is isotopically light, with δ¹³C values of about −21 to −34 per mil, and the paired carbon and nitrogen isotopes plot in a field distinct from conventional diamonds[1]. Its photoluminescence spectra resemble those of irradiated and heated vapor-deposited diamond[1]. Its inclusions are not the garnet, pyroxene and olivine of mantle diamonds but metals, metal alloys, moissanite (silicon carbide) and osbornite (titanium nitride), phases stable only under strongly reducing conditions such as those deep in the mantle or in space[1].
Each of these observations has a terrestrial rejoinder. Ishibashi and colleagues argued that hydrous fluids, rather than interstellar gas, were the growth medium[7]; a 2008 spectroscopic study of Central African carbonado attributed infrared bands to water and carbonate trapped in the diamond[8]. Daulton and Ozima showed that radiation from uranium can convert carbonaceous material to nanodiamond[9], and Smith and Dawson proposed impacts into carbon-bearing crust[10]. Haggerty himself catalogues five families of explanation—impact, growth in the crust or mantle, subduction, radiation and an extraterrestrial origin—and concedes that “none are uniformly accepted”[1].
A widely cited review in Elements by Peter Heaney, Edward Vicenzi and Subarnarekha De frames the puzzle from the opposite side. Its authors note that, in contrast to kimberlitic diamonds, the geochemical signatures of carbonado are “systematically crustal,” and that its apparent age is Archean, about 3.2 billion years[11]. Those two facts, they observe, have driven investigators toward exotic explanations: metamorphism of the earliest subducted lithosphere, radioactive transformation of hydrocarbons, and meteorite impact on concentrated biomass[11]. The stellar model is one member of that family of unconventional answers, not the only one.
The stellar hypothesis, then, is best read as an argument from exclusion supported by suggestive spectra, not as a demonstrated history. Haggerty states the test plainly: the model “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”[1]. No such discovery has yet been reported in the sources cited here.