Carbonado, or black diamond, is one of the toughest and most enigmatic forms of natural diamond. Unlike conventional, sparkling gemstones, these opaque, dark stones are polycrystalline aggregates of carbon—a mass of countless microscopic crystals clumped together. Their unusual color, unique physical properties, and limited geographical distribution have made their formation one of geology’s most enduring puzzles. Evidence suggests the origin of black diamonds does not align with the accepted model for other diamonds, pointing instead to a history far removed from the Earth’s mantle.
The Unique Crystalline Structure of Black Diamonds
The structure of Carbonado is fundamentally different from that of a clear, single-crystal diamond. Rather than being one continuous crystal lattice, Carbonado is a fine-grained, sintered aggregate composed of diamond, graphite, and amorphous carbon. This polycrystalline texture gives the stone extreme durability, making it much tougher than a monocrystalline diamond. A defining characteristic is its high porosity, with void spaces often ranging between 5% and 15% of the total volume. These microscopic pores contribute to the stone’s opaque, charcoal-like appearance, which is the origin of its name from the Portuguese word for “burned.”
Carbonado also contains unique chemical signatures, including trace elements of nitrogen and hydrogen present interstitially within the crystal structure. Some carbonados contain exotic mineral inclusions such as osbornite (titanium nitride) and various metal alloys. These inclusions are typically found only in meteorites and other extra-terrestrial materials.
Standard Diamond Formation Versus Carbonado
Conventional diamonds, which are typically translucent or clear, are born deep within the Earth’s mantle, more than 150 kilometers below the surface. Their formation requires immense pressure and temperatures often exceeding 1,000 degrees Celsius. These mantle-derived diamonds are then rapidly carried to the surface through volcanic eruptions, becoming embedded in igneous host rocks like kimberlite or lamproite.
Carbonado adheres to none of these geological rules. Black diamonds are never found within kimberlite or lamproite pipes, nor do they contain the mineral inclusions characteristic of the Earth’s mantle. Instead, they are found exclusively in younger sedimentary layers known as alluvial deposits, suggesting they were eroded from an unidentified source rock. The light carbon isotope values found in Carbonado are also distinctly different from the heavier isotopes typical of mantle-derived diamonds, further indicating a non-traditional origin.
The Extraterrestrial Origin Hypothesis
The most compelling theory for the formation of Carbonado is that the material originated in space billions of years ago. This hypothesis suggests that black diamonds were first created in the hydrogen-rich environment of interstellar space, possibly within the remnants of a supernova explosion. The shockwaves and immense forces generated by a stellar collapse could have provided the necessary energy to form the diamond aggregates.
The unique chemical composition of the stones provides evidence for this cosmic birthplace. Analysis has confirmed the presence of hydrogen and nitrogen, which are signatures of a hydrogen-rich environment like that found in space. The carbon isotopes within Carbonado also show a striking similarity to those found in diamonds within meteorites, lending credence to a shared cosmic heritage.
Scientists propose that these diamond aggregates coalesced into large masses, perhaps a kilometer or more in diameter, before eventually crashing into Earth. This impact event, or barrage of events, occurred approximately 2.3 to 4 billion years ago, when the Earth was still subject to heavy meteor bombardment. The sheer scale and velocity of the impact would have scattered the diamond material across the continents, explaining its unusual distribution in surface deposits today.
Geographic Distribution and Geological Context
The geological context of Carbonado is unusual, as the stones are found in only two locations: Brazil and the Central African Republic (CAR). In both regions, the diamonds are recovered from ancient alluvial and sedimentary deposits, such as stream beds and gravels, rather than a primary volcanic source.
This geographically restricted distribution supports the extraterrestrial impact theory. Brazil and the CAR were once part of the same supercontinent, which existed until about 180 million years ago. A single, massive asteroid impact billions of years ago would have showered the now-separated landmasses with the diamond material.