Black diamonds, often referred to as “carbonado,” are a unique class of diamond material. Unlike the colorless, single-crystal stones typically associated with fine jewelry, a black diamond is an aggregate of millions of microscopic diamond crystals fused together. This composition results in properties that set them apart from their transparent counterparts, including their durability, porosity, and intense, opaque color.
The Material Causing the Color
The deep, opaque black color is not an inherent property of the carbon lattice itself, unlike the nitrogen that causes yellow in traditional diamonds. Instead, the color is caused by vast quantities of dark, opaque mineral inclusions trapped within the crystal structure. These micro-inclusions are so numerous and densely distributed that they prevent virtually all light from passing through the stone, resulting in the completely black and often metallic-looking appearance.
The specific materials responsible for this coloration are typically various iron oxides and forms of carbon other than diamond. These include graphite, which is an opaque form of carbon, and iron-bearing minerals like hematite and magnetite. The presence of these mineral inclusions can sometimes give the diamonds an unusually high magnetic susceptibility, which helps verify their natural origin.
Physical Structure and Properties
The internal arrangement of a black diamond, particularly the natural variety known as carbonado, is fundamentally different from a typical gem-quality diamond. Colorless diamonds are “monocrystalline,” meaning they are a single, continuous crystal lattice of carbon atoms. Black diamonds, conversely, are “polycrystalline aggregates,” composed of countless minute diamond grains, amorphous carbon, and graphite sintered together.
This aggregated structure gives the material unique physical properties. The random orientation of the tiny crystals results in a material that is extremely tough and resistant to breakage, even though its hardness is the same as a normal diamond. However, this structure also makes the material slightly porous and exceptionally difficult to cut and polish. The porosity is a characteristic feature of carbonados, with void space sometimes accounting for 6% to 13% of the stone’s volume.
Mysterious Origins and Formation
The formation of natural black diamonds is one of the most debated topics in diamond geology, as they do not conform to the accepted model for traditional diamond creation. Most diamonds form deep within the Earth’s mantle under extreme pressure and temperature, later brought to the surface by kimberlite volcanic pipes. Carbonados, however, are never found in these kimberlite sources; they are only recovered from alluvial, sedimentary deposits.
Scientists have proposed several theories to explain this discrepancy, with the most dramatic suggesting an extraterrestrial origin. This theory posits that carbonados were formed in the hydrogen-rich environment of space, potentially in the remnants of a supernova explosion. According to this hypothesis, the material then fell to Earth as large meteorites approximately 2.3 billion years ago. Supporting this idea is the presence of trace elements and carbon isotope ratios in carbonados that are similar to those found in meteorites.
A compelling piece of evidence for their unique origin is their restricted geographic distribution. Natural carbonados have only been found in two regions on Earth: Brazil and the Central African Republic. This limited occurrence aligns with the idea that a single, large impact event occurred when these two landmasses were joined as part of a supercontinent. The lack of typical mantle mineral inclusions continues to point toward a non-traditional genesis.
Natural Versus Enhanced Black Diamonds
The vast majority of black diamonds in the jewelry market today are not naturally occurring carbonados; they are natural diamonds that have been color-enhanced through laboratory treatment. These treated stones begin as heavily included, low-quality diamonds, often gray, brown, or dark green, chosen because their initial flaws make them unsuitable for use as colorless gems.
The enhancement process involves subjecting the diamonds to high-temperature/low-pressure (HTLP) treatment or irradiation. These processes heat the inclusions within the diamond, often causing them to graphitize, or transform into dark, opaque carbon. The treatment effectively darkens the existing fractures and inclusions, turning the entire stone a uniform, opaque black. This allows jewelers to offer a consistently colored stone at a much lower cost than the natural carbonado.
A natural carbonado is opaque, often slightly porous, and extremely rare, fetching a premium price. Conversely, an enhanced black diamond is more common and less valuable, though its color is typically more uniformly distributed and intense. Reputable sellers are required to disclose these treatments, as the enhancement significantly affects the stone’s market value and classification.