Black diamonds, known scientifically as carbonado, stand apart from their colorless, transparent counterparts. While conventional diamonds form deep within the Earth’s mantle, carbonado’s origin has long been a mystery, with its distinct appearance hinting at a non-terrestrial genesis. These opaque, dark stones are exceptionally rare, found primarily in alluvial deposits in only two regions: the Central African Republic and Brazil. Their unique chemical and structural makeup suggests they arrived on Earth from space.
Structural Composition of Black Diamonds
Black diamonds are fundamentally different from traditional gem-quality diamonds, which are single, large crystals. Instead, carbonado is a polycrystalline aggregate, meaning it is composed of countless microscopic diamond crystals cemented together in a dense mass. These tiny diamond grains, often measuring only a few micrometers in size, are bound by a matrix of other carbon forms, including graphite and amorphous carbon.
This aggregated structure gives carbonado its characteristic opaque, dark color and matte luster. The black appearance is not due to the diamond itself, but rather to numerous inclusions of graphite, sulfides, and iron oxides like magnetite or hematite scattered throughout the matrix. These dark mineral inclusions absorb light, preventing the brilliance seen in clear diamonds.
A defining feature of carbonado is its high porosity, with void spaces ranging from about 6% to 13% of its volume. This micro-porous texture, which makes the stone look somewhat like a sponge, is a major clue ruling out traditional high-pressure, high-temperature formation deep within the Earth’s mantle. Such high-pressure environments do not allow for the gases necessary to create this bubbly, porous structure.
The Extraterrestrial Formation Hypothesis
The unique composition and structure of black diamonds, coupled with their restricted geographical distribution, strongly suggest an origin outside of the Earth’s interior. The leading scientific theory proposes that carbonado formed in space and was delivered to Earth via a massive impact event billions of years ago. This hypothesis contrasts sharply with the formation of conventional diamonds, which crystallize from carbon-bearing fluids in the Earth’s mantle at depths greater than 90 miles.
One compelling piece of evidence supporting this extraterrestrial origin is the presence of noble gases and specific isotopic signatures trapped within the carbonado structure. The nitrogen and hydrogen trace elements found within the micro-diamonds match the signatures of cosmic sources and diamonds found in meteorites. This indicates formation in a hydrogen-rich environment, such as interstellar space, rather than the Earth’s mantle.
The proposed mechanism involves the formation of carbonado inside a supernova, the explosion of a giant star, which scattered the material across the cosmos. After coalescing into a large body, possibly an asteroid a kilometer or more in diameter, the carbonado mass is thought to have impacted Earth approximately 2.3 billion years ago. The impact site, which was a single landmass at the time, later separated to form the regions of Brazil and the Central African Republic.
The carbon isotope values of carbonado are also lighter than those found in terrestrial diamonds, further distinguishing their carbon source as non-mantle. The combination of these chemical fingerprints with the unusual porous, polycrystalline structure makes the extraterrestrial hypothesis the most comprehensive explanation. This theory moves the time of their crystallization back to the deep time before the solar system existed.
Unique Physical Properties and Industrial Use
The polycrystalline structure of carbonado directly influences its physical properties, making it suited for industrial applications. While all diamonds have a Mohs hardness of 10, the aggregated nature of the black diamond makes it significantly tougher than a single-crystal diamond. Toughness refers to a material’s resistance to fracture, and the many small crystals prevent cracks from propagating easily along a single cleavage plane.
This enhanced toughness means carbonado is less prone to catastrophic cleavage and wear, making it more valuable as an abrasive than other diamond varieties. The aggregated texture allows an abrasive tool to present multiple hard crystal orientations at the cutting surface, increasing its effectiveness. Since the late 19th century, this material has been used extensively in cutting tools, drill bits, and grinding applications.
Black diamond coatings on drill bits provide exceptional durability for working with challenging materials, including ceramics and composites. Beyond mechanical uses, carbonado’s high thermal conductivity also makes it valuable in high-tech applications, such as heat sinks and thermal management systems for electronics. Though challenging to cut and polish, black diamonds are also used in jewelry for their unique, opaque aesthetic.