Carbonados, often called "black diamonds," are a significant geological mystery, differing from clear diamonds in being dark, porous, and unusually tough, with a unique history and intensely debated origins.
They were first discovered in Brazil's Bahia region in 1841. Portuguese prospectors named them carbonado, meaning carbonized or burned, due to their charcoal-like appearance. They were later found in the Central African Republic, which, along with Brazil, are the only two locations, leading geologists to believe they were deposited over three billion years ago before the Gondwana supercontinent split, suggesting a single massive event distributed them.
Due to their polycrystalline structure—composed of many tiny crystals fused together—carbonados lack the cleavage planes of traditional diamonds, making them significantly tougher, and they were historically used as industrial abrasives, including in the construction of the Panama Canal.
Multiple competing theories exist regarding their origin, as they are never found in kimberlite pipes, the traditional volcanic sources for diamonds.
One prominent explanation, popularised by Stephen Haggerty, is the extraterrestrial theory or the supernova model, which posits that carbonados formed in the carbon-rich environment of a supernova explosion billions of years ago, later traveling through space as a large asteroid and impacting the Brazil-Africa landmass approximately 2.3 to 3.2 billion years ago; this is supported by the presence of hydrogen and nitrogen in ratios matching interstellar space rather than the Earth's mantle, and by the rare mineral osbornite, commonly found in meteorites.
-
The Idea: Carbonados formed in the carbon-rich environment of a supernova explosion billions of years ago. They then traveled through space as a large asteroid and crashed into Earth (impacting the Brazil/Africa landmass) roughly 2.3 to 3.2 billion years ago.
-
The Evidence: They contain trace elements like hydrogen and nitrogen in ratios that match interstellar space rather than the Earth's mantle. They also contain osbornite, a rare mineral found in meteorites.
Another possibility is the terrestrial impact theory, suggesting a large meteorite struck a carbon-rich area on Earth's surface, such as a peat bog or coal bed, where the intense heat and pressure of impact instantly converted that terrestrial carbon into polycrystalline diamonds, although known impact diamonds, like those at the Popigai crater in Russia, typically have a different crystal structure and size than carbonados.
-
The Idea: A large meteorite hit a carbon-rich area on Earth's surface (like a massive peat bog or coal bed). The intense heat and pressure of the impact instantly converted that terrestrial carbon into polycrystalline diamonds.
- The Evidence: Impact diamonds (like those in the Popigai crater in Russia) exist, but they are usually much smaller and have a different crystal structure (lonsdaleite) than carbonados.
A third theory proposes radiation-induced formation within the Earth's crust, where the radioactive decay of uranium and thorium bombarded organic carbon over millions of years, forcing the atoms into a diamond structure, and while some carbonados are found in uranium-rich regions, critics argue radiation alone lacks sufficient energy to create stones as large as those found.
-
The Idea: Carbonados formed within the Earth's crust through the radioactive decay of uranium and thorium. Over millions of years, the radiation bombarded organic carbon (like shungite or hydrocarbons), forcing the atoms into a diamond structure.
- The Evidence: Some carbonados are found in uranium-rich regions, but critics argue that radiation alone couldn't provide enough energy to create stones as large as the ones we've found (like the 555-carat "Enigma").
Finally, the deep earth subduction theory posits that organic matter from the Earth's surface was swallowed by tectonic plates and pushed deep into the mantle where it crystallized into diamonds before being pushed back up, but this theory faces challenges explaining the significant porosity of carbonados, as the intense mantle pressure would typically crush any bubbles or pores in a gemstone. The debate over carbonados remains active because they are a geological anomaly that does not fit the rules of standard diamond mineralogy and are found only in alluvial river deposits rather than their original birth rock, leaving scientists without definitive proof of their origin.
-
The Idea: Organic matter from the Earth's surface was "swallowed" by tectonic plates (subduction) and pushed deep into the mantle where it crystallized into diamonds before being pushed back up.
-
The Evidence:: this theory rests primarily on isotopic signatures and inclusions within the carbonados.
-
Light Carbon Isotopes: Carbonados are often found to have very low delta-C-13 values. These 'light' carbon isotope values are characteristic of biological matter found on Earth's surface (like ancient marine life or organic sediments). While mantle carbon has a distinct isotopic range, organic material has a much higher concentration of the lighter carbon-12 isotope, leading to these low values. Finding this signature in carbonados suggests that their carbon source was originally from the crust and was later subducted.
-
Unusual Mineral Inclusions: When analyzed under intense magnification, some carbonados show inclusions of minerals that are not typical for standard mantle-derived diamonds. These inclusions, such as minerals rich in chromium or potassium, have compositions that suggest they may have originated in lower-pressure, crustal-like environments before being subjected to ultra-high pressure during subduction and incorporated into the forming diamond.
To understand this process, look at a cross-section of a subduction zone. Here, the oceanic lithosphere (the dense, rigid outer layer of the Earth, often carrying carbon-rich sediments) descends into the mantle beneath an overriding plate. This descending slab can transport surface-derived organic carbon to extreme depths (hundreds of kilometers). As the material descends, it experiences tremendous heat and pressure. The diagram illustrates how, as the slab moves deeper into the mantle, it crosses the 'diamond stability field.' This region, starting below approximately 140 kilometers, is where carbon, under appropriate conditions, can be converted into the diamond crystal structure. Proponents of Theory 4 argue that this provides a mechanism to bring crustal organic carbon to the specific pressure and temperature window required for diamond formation, while also explaining the unique geochemical signatures found in carbonados.
