Black Diamond
Black Diamond
Opaque, iron-dark, and structurally distinct — the diamond world's most unconventional variety
Black diamond occupies a singular position among diamond varieties: it is the only commercially significant form of the mineral that is entirely opaque, and it encompasses two fundamentally different geological phenomena under a single trade name. The first is heavily included single-crystal diamond whose colour derives from dense clouds of dark mineral inclusions — principally graphite, hematite, or magnetite — distributed throughout the stone. The second is carbonado, a polycrystalline aggregate of microcrystalline diamond grains bonded by a porous, amorphous carbon matrix, found exclusively in Brazil and the Central African Republic. Both types have been fashioned into jewellery, particularly since the late 1990s when the black diamond aesthetic gained broad commercial traction, yet they differ so profoundly in origin, structure, and optical behaviour that gemmologists treat them as distinct materials sharing a colour category rather than a single gem variety.
Gemological Characteristics
Single-crystal black diamonds share the fundamental chemistry and crystal structure of colourless diamond — pure carbon in the cubic system, Mohs hardness 10, specific gravity approximately 3.52 — but their optical properties diverge sharply from transparent gem-quality material. The extreme density of inclusions suppresses all transmission of light, rendering the stone fully opaque. Refractive index measurements are practically impossible on finished stones because light does not pass through the material in any meaningful way. Surface lustre, when well polished, is an intense adamantine to submetallic sheen that distinguishes natural black diamond from black glass or black spinel.
Carbonado differs structurally at every level. Rather than a single crystal, it is a sintered aggregate of diamond crystallites typically ranging from a few nanometres to several micrometres in size, interspersed with graphite, amorphous carbon, and a variety of mineral inclusions including fluorapatite, forsterite, and iron-bearing phases. Its specific gravity is lower than single-crystal diamond, typically 3.1–3.5, owing to its inherent porosity. Carbonado is also notably tougher in the practical sense — the absence of continuous cleavage planes across the polycrystalline mass means it resists fracture better than single-crystal diamond along certain orientations — which historically made it prized for industrial drilling and cutting applications before synthetic alternatives became dominant.
Colour Causes
In single-crystal black diamonds, colour is not a lattice-level phenomenon as it is in blue, yellow, or pink diamonds. No specific atomic defect or impurity produces black colour in isolation. Instead, the opacity and blackness arise from the cumulative absorption of light by vast numbers of dark inclusions — most commonly graphite (which may represent partial or complete graphitisation of the host diamond lattice), hematite, and magnetite. The inclusions are so numerous and so finely distributed that they collectively absorb all wavelengths of visible light, leaving no spectral signature to transmit. This inclusion-driven mechanism means that the colour is, in a sense, a structural accident rather than an intrinsic property of the diamond itself.
The origin of carbonado's dark colour is similarly tied to its carbon phases: graphite and amorphous carbon within the matrix absorb light comprehensively. Some carbonado specimens display surface colours ranging from dark grey to brownish-black rather than a pure jet black, reflecting variations in the ratio of diamond to graphitic carbon within the aggregate.
Geographic Origins
Natural single-crystal black diamonds are recovered from alluvial and primary kimberlite deposits worldwide, including sources in Africa, Russia, and Australia, though they are seldom specifically documented by locality in the trade because their value is not strongly origin-dependent in the way that coloured sapphires or rubies are. The stones are typically recovered as a by-product of conventional diamond mining, having been rejected for gem use due to their inclusions before the black diamond market developed.
Carbonado is geographically anomalous and scientifically controversial. Commercial quantities are known from only two regions: the state of Bahia in Brazil and the Ubangi region of the Central African Republic. The two occurrences are separated by the South Atlantic Ocean, and both are found in ancient Precambrian sedimentary and alluvial contexts with no associated kimberlite pipes — the usual source of diamond. This distribution, combined with carbonado's unusual isotopic signatures and the presence of certain mineral inclusions, has prompted competing hypotheses for its origin, including deep-mantle formation, subduction-zone processes, and — more controversially — an extraterrestrial impact or interstellar origin. No scientific consensus has been reached, and the origin of carbonado remains one of the more genuinely unresolved questions in mineralogy.
Treatment
The majority of black diamonds sold in the contemporary jewellery market are not naturally black. They are heavily included grey, dark grey, or brownish diamonds — material that would otherwise be of very low gem value — that have been subjected to irradiation followed by high-temperature annealing to produce a uniform, stable black colour. The irradiation (typically using a linear accelerator or nuclear reactor) creates colour centres and promotes graphitisation of existing fractures and inclusions, darkening the stone throughout. Subsequent heat treatment stabilises the colour and removes unwanted greenish or brownish overtones.
Treated black diamonds are considered permanent under normal wearing conditions: the colour does not fade with light exposure or ordinary heat. However, because the treatment exploits and deepens existing fractures and inclusions, treated stones are often more fragile than their hardness rating implies — fractures that have been graphitised may propagate under impact. Disclosure of treatment is required by major gemmological laboratories and trade organisations. Gemological Institute of America (GIA) reports on black diamonds specify whether the colour is natural or the result of treatment, a distinction that carries significant price implications.
A small number of black diamonds are also coated — a surface film of black material applied to a near-colourless or grey diamond — though this is a far less stable and less prevalent treatment than irradiation. Coating can be detected by examining the girdle and facet junctions under magnification.
Identification and Laboratory Reports
Distinguishing natural-colour black diamond from treated black diamond, and both from simulants such as black spinel, black tourmaline, black glass, or jet, requires a combination of techniques. Specific gravity measurement, hardness testing, and examination under magnification are primary tools. Infrared spectroscopy can reveal the diamond's nitrogen aggregation state and, in some cases, provide evidence of irradiation treatment. Raman spectroscopy is particularly useful for confirming the presence of diamond and detecting graphitic carbon phases. GIA and other major laboratories issue grading reports for black diamonds that document colour origin; the absence of a laboratory report from a reputable institution should be considered a significant caveat when purchasing any black diamond represented as naturally coloured.
Notable Specimens
The most celebrated black diamond is the Spirit of de Grisogono, a 312.24-carat polished black diamond of West African origin, cut by the Geneva jeweller de Grisogono and set into a white gold ring surrounded by white diamonds. It is among the largest cut black diamonds known. The Black Orlov, a 67.50-carat cushion-cut stone with a colourful provenance narrative, is another frequently cited example, though the historical claims attached to it are largely unverified. The Gruosi Diamond, a 115.34-carat heart-shaped black diamond, was cut by de Grisogono from a 300-carat rough stone recovered in Africa. These stones demonstrate the scale at which natural black diamond rough can occur — a consequence of the fact that the inclusions that produce black colour do not impede crystal growth as they would compromise transparency in a gem-quality stone.
In the Trade
Black diamonds entered mainstream jewellery consciousness in the late 1990s and early 2000s, driven in part by their adoption by avant-garde designers seeking a bold, graphic aesthetic and by their relative affordability compared to colourless or fancy-colour diamonds of equivalent carat weight. Treated black diamonds in particular are priced at a fraction of natural-colour stones: commercial treated material of one carat may trade at prices comparable to low-quality colourless melee, while a certified natural-colour black diamond of significant size commands a premium reflecting its rarity and the difficulty of confirming natural colour origin.
The market distinguishes clearly — or should distinguish clearly — between three commercial categories: natural-colour black diamond, treated black diamond, and carbonado. Carbonado is rarely seen in fine jewellery; its porous, irregular structure makes polishing difficult and setting precarious, though a small number of designer jewellers have incorporated rough or partially polished carbonado as a deliberate textural element. Industrial-grade carbonado, once a significant commodity for drill bits and abrasive tools, has been largely supplanted by synthetic polycrystalline diamond produced under controlled conditions.
When purchasing black diamonds, buyers should request laboratory documentation specifying colour origin, examine the stone for surface-reaching fractures that may compromise durability, and be aware that the adamantine lustre of a well-polished natural or treated black diamond is one of its principal aesthetic virtues — stones with a dull or waxy surface finish may indicate poor polishing, coating, or a non-diamond simulant.