The natural black variety of diamond, scientifically known as Carbonado, is an opaque aggregate of millions of tiny crystals fused together. Unlike the transparent, single-crystal structure of a traditional diamond, these unusual stones are extremely rare. They have only been found in alluvial deposits in two specific regions of the world: Brazil and the Central African Republic.
What Black Diamonds Are
Carbonados are classified as polycrystalline diamonds, meaning they are composed of many microscopic diamond grains bonded together. This structure contrasts sharply with the monocrystalline nature of colorless diamonds, which are single, large, continuous crystals. The aggregate composition results in a highly dense, yet micro-porous material.
This distinct structure makes Carbonado one of the toughest forms of diamond, even though its hardness on the Mohs scale remains a ten. The lack of a single cleavage plane means the stone is much more difficult to cut and polish than a traditional diamond.
How They Get Their Color
The intense black hue of the Carbonado does not come from trace elements within the diamond’s lattice. Instead, the color is a result of dense concentrations of dark mineral inclusions scattered throughout the stone’s porous matrix, primarily composed of graphite and amorphous carbon.
These embedded particles are so numerous and uniformly distributed that they prevent light from passing through or being refracted. The stone absorbs all the light that hits its surface, giving it a deep, non-reflective, and matte-to-metallic black appearance.
Competing Theories of Origin
The geological origin of Carbonado is a major puzzle in diamond science, as they do not follow the formation rules of traditional diamonds. The scientific debate centers on two main competing theories: terrestrial and extraterrestrial. The terrestrial theory suggests Carbonado formed deep within the Earth’s mantle, similar to other diamonds, but under highly unusual conditions.
The extraterrestrial hypothesis proposes that black diamonds were formed in space, perhaps within the intense pressure environment of a supernova explosion. They were then delivered to Earth’s surface via an asteroid or meteorite impact approximately 2.3 to 3.8 billion years ago.
Evidence for Extraterrestrial Origin
Evidence for this cosmic origin includes the presence of trace noble gases and hydrogen within the stones, elements typically associated with interstellar space. The limited geographical distribution of Carbonado (only in Brazil and the Central African Republic) is consistent with a single, massive impact event that occurred when those landmasses were still joined as part of the supercontinent Gondwana. The lack of their discovery in kimberlite pipes, the conventional source for all other diamonds, further supports a non-traditional formation history.
Distinguishing Natural and Treated Stones
Most black diamonds sold in the commercial jewelry market are color-treated stones. These enhanced diamonds begin as lower-quality, heavily included, or fractured gray and colorless diamonds. They are then subjected to High-Pressure/High-Temperature (HPHT) treatment or irradiation.
The goal of the treatment is to permanently darken the stone by heating it to convert the existing internal fractures and inclusions into black graphite. Natural black diamonds are entirely opaque and often exhibit surface porosity, a characteristic of their polycrystalline structure. In contrast, treated black diamonds sometimes show a concentration of color along surface-reaching fractures or may display a deep, dark green body color when viewed under strong transmitted light, a remnant of the irradiation process used to achieve the black color.