Carbonado was discovered in Brazil in 1841 and named by Portuguese prospectors for its resemblance to charcoal[1]. It was later found in the Central African Republic[1]. These remain its only known sources[2,1], and it occurs in alluvial and placer deposits, never in kimberlite[3].
Its toughness made it valuable to industry. The French prized it as a polishing material, it was used for drilling during construction of the Panama Canal, and it formed part of the U.S. strategic mineral stockpile as recently as 1990[1]. At the height of alluvial mining in Brazil, from 1850 to 1870, an estimated 30,000 artisanal miners produced some 70,000 carats[1].
Haggerty examined about 800 carbonados from both countries and found no significant differences in texture, appearance or physical and chemical properties[1], which supports a common source.
The origin remains contested. Published proposals include meteorite impact[4], growth from fluids in the crust or mantle[5,6], subduction, radiation-induced formation in uranium-rich carbon[7] and an extraterrestrial origin[8,9]. Haggerty’s review weighs each and finds none uniformly accepted[1].
His objections are specific. An impact into organic-rich crust would require several cubic kilometers of uncontaminated carbon to yield the roughly two tons of carbonado recovered so far, and known impact diamonds are discrete microdiamonds[1]. Crustal or mantle fluids capable of lowering diamond’s stability limits would be hydrous and strongly oxidized, conditions in which diamond is unlikely to crystallize or survive[1]. Subduction would leave mantle minerals such as garnet and pyroxene, and none is observed[1]. Radiation produces diamond only at the nanometer scale, far smaller than carbonado’s crystals[1].
Proponents of terrestrial models reply with their own data: crustal geochemical signatures[10], water and carbonate inclusions[6], and experiments on radiation-induced diamond[7]. The disagreement is not between evidence and speculation but between different readings of the same unusual material.