VarietyA variety of Diamond

Carbonado

CKarbonado

A black, porous natural diamond made of countless tiny crystals: a tough drilling material whose origin is still debated, also called “black diamond”.

Also known as: Black diamond · Carbon (trade name for Central African carbonado)

Popularly: Ore and raw material

Photo: James St. John · CC BY 2.0 · Wikimedia Commons

In short

Carbonado is a black or dark grey, porous natural diamond made not of one large crystal but of countless tiny diamond crystals firmly bonded together. Portuguese diamond prospectors named it for its resemblance to charcoal; in English it is also called “black diamond”. Almost all known carbonado comes from alluvial deposits (river gravels) in the Brazilian state of Bahia and in the Central African Republic. It is as hard as diamond but much tougher, which is why it was long used in drill bits. How it formed is one of science’s unsolved questions.

Photos

Carbonado specimen (2) — Photo: SixFingeredMan · Public domain · Wikimedia Commons

How does it form?

How carbonado forms is not known for certain. Ordinary diamonds form about 150–200 kilometres below the surface and are carried up by volcanic rocks such as kimberlite; yet among the roughly 700 tonnes of diamond mined since about 1900, not a single carbonado has been reported from kimberlite. Carbonado also lacks mantle minerals such as olivine, garnet or pyroxene. Its inclusions give ages between 2.6 and 3.8 billion years.

Many explanations have therefore been proposed: the shock of a meteorite impact, the conversion of organic carbon in the crust under high pressure, the effect of radioactive elements such as uranium and thorium, or formation in space, in carbon-rich stars or planets, followed by delivery to Earth in meteorites. The geologist Stephen Haggerty argues for an origin in space, pointing out that the pores are incompatible with high-pressure environments and that carbonado contains osbornite (titanium nitride), a mineral also found in meteorites. Other researchers point to similarities with Earth’s diamonds; the debate continues.

What is it used for?

Drilling and cutting: Because its tiny crystals point in random directions, carbonado does not split along a plane as a single diamond crystal does, and it is extremely tough. Since the 19th century it has been used as an abrasive, in drill bits for the hardest rocks and for polishing.
Jewellery and collecting: It is so hard and tough that it can only be cut by laser, so polished carbonado is rare. A 555.55-carat black carbonado with 55 facets, called the “Enigma”, was sold at auction.
Science: Because its origin is unknown, carbonado is studied by researchers who want to understand Earth’s early history and the settings in which diamond can form.

Where is it found?

Carbonado is found only in alluvial deposits, the gravels and sands laid down by rivers. The Lençóis area in the Chapada Diamantina of Bahia, Brazil, was the world’s only commercial source until the early 20th century; at the height of mining, between 1850 and 1870, an estimated 30,000 miners worked there. As the Bahia deposits ran out, the diamond fields of the Central African Republic came into production from around 1925. In that country diamonds are mined mainly in two areas: around Berbérati, Carnot and Nola in the west, and around Ouadda and N’Délé in the north-east. The western carbonados are large and can make up as much as 30 percent of diamond production; those in the east are small. The two regions, now separated by an ocean, were part of the same supercontinent for more than a billion years.

What determines its value?

For a long time carbonado’s value came from its industrial role: being hard and resistant to breaking made it a prized material for drilling and polishing. Today its value depends more on size, soundness and rarity; but because it can only be cut by laser, there are very few large polished carbonados, and stones with a known history are collector’s pieces. A laboratory report matters so that it is not confused with heat-treated single-crystal black diamonds or with black imitations.

This information is educational, not buying, selling or investment advice. Value varies enormously with quality and treatment.

How to recognise it

1
It is not transparent; it is black, dark grey or brown, sometimes with purple, pink, rusty red or olive green tones.
2
Its surface shows pores visible to the eye or under a loupe, and a glassy coating that looks as if it had melted.
3
Being diamond, it scratches everything, but because it is porous it is lighter than single-crystal diamond: its density is about 2.8–3.45 g/cm³, against 3.52 g/cm³ for single-crystal diamond.
4
Almost all known carbonado comes from alluvial deposits in Brazil and the Central African Republic; none has ever been reported from kimberlite, the rock that carries most diamonds.

Fakes and imitations

What shops usually don’t tell you:

Heat-treated black diamonds: heavily fractured colourless or grey diamonds are heated so that the carbon in their cracks turns to graphite and the stone looks black. These are single crystals; they should not be confused with carbonado, and the treatment should be disclosed.
Natural-colour single-crystal black diamonds: they owe their colour to dark inclusions such as graphite, sulfides or iron minerals, and they too are called “black diamonds”. Unlike carbonado they are not porous; a gemmological laboratory is needed to tell them apart for certain.

Myths and misconceptions

MythCarbonado has been proven to come from outer space.
FactSome researchers argue that carbonado formed in carbon-rich stars or planets and reached Earth in meteorites; others think it formed deep inside the Earth. No single model explains everything, and its origin is still debated.
MythAll the black diamonds seen in jewellery are carbonado.
FactMost black diamonds in jewellery are single-crystal diamonds: they owe their colour to dark inclusions such as graphite and sulfides, or turn black when heavily fractured diamonds are heated. Carbonado is a separate, porous, polycrystalline form of diamond and is very hard to cut.

Did you know?

The largest known carbonado, the “Sergio”, was found near Lençóis in Bahia; it weighed more than 3,000 carats, heavier even than the rough Cullinan, the largest single-crystal diamond. The Sergio was broken up into small pieces for drill bits.

At the beginning of the 20th century carbonado was more expensive by weight than diamond; it was used for drilling during the construction of the Panama Canal and was part of the US strategic mineral stockpile as recently as 1990.

According to a paper published in 1971, diamond dealers kept the name “carbonado” for Brazilian stones and called the Central African material “carbon”. The same paper reports that the largest known Central African carbonado weighed 740.25 carats and was on display at the Smithsonian Institution.

Where is it found?

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Lençóis (Chapada Diamantina), Bahia, Brazil · Major depositUntil the early 20th century Bahia was the only commercial source of carbonado; the largest known carbonado, the “Sergio”, was found here
Berbérati area, Mambéré-Kadéï, Central African Republic · Major depositWest Ubangi diamond fields: up to 30 percent of the diamonds here are large carbonados
Carnot area, Mambéré-Kadéï, Central African Republic · Major depositWest Ubangi diamond fields
Nola area, Sangha-Mbaéré, Central African Republic · Known localityWest Ubangi diamond fields
Ouadda area, Haute-Kotto, Central African Republic · Known localityEast Ubangi diamond fields: carbonados are small and make up 6–7 percent of production
N'Délé area, Bamingui-Bangoran, Central African Republic · Known localityEast Ubangi diamond fields

Locations are approximate to protect sites. Red dots mark places in Türkiye. Open in the big map →

Frequently asked questions

What is carbonado?

What is carbonado used for?

Drilling and cutting: Because its tiny crystals point in random directions, carbonado does not split along a plane as a single diamond crystal does, and it is extremely tough. Since the 19th century it has been used as an abrasive, in drill bits for the hardest rocks and for polishing. Jewellery and collecting: It is so hard and tough that it can only be cut by laser, so polished carbonado is rare. A 555.55-carat black carbonado with 55 facets, called the “Enigma”, was sold at auction.

How hard is carbonado?

It has a hardness of 10 on the Mohs scale. It scratches glass and steel; a steel knife cannot scratch it. Its specific gravity is 2.8–3.45 g/cm³.

Where is carbonado found?

Its best-known localities: Lençóis (Chapada Diamantina), Bahia, Brazil; Berbérati area, Mambéré-Kadéï, Central African Republic; Carnot area, Mambéré-Kadéï, Central African Republic; Nola area, Sangha-Mbaéré, Central African Republic; Ouadda area, Haute-Kotto, Central African Republic.

How can you tell if carbonado is real?

It is not transparent; it is black, dark grey or brown, sometimes with purple, pink, rusty red or olive green tones. Its surface shows pores visible to the eye or under a loupe, and a glassy coating that looks as if it had melted. Being diamond, it scratches everything, but because it is porous it is lighter than single-crystal diamond: its density is about 2.8–3.45 g/cm³, against 3.52 g/cm³ for single-crystal diamond. Heat-treated black diamonds: heavily fractured colourless or grey diamonds are heated so that the carbon in their cracks turns to graphite and the stone looks black. These are single crystals; they should not be confused with carbonado, and the treatment should be disclosed.

Is carbonado valuable?

On the “street to diamond” value ladder it sits on step 6 of nine: Fine ornamental and collector stone. For a long time carbonado’s value came from its industrial role: being hard and resistant to breaking made it a prized material for drilling and polishing. Today its value depends more on size, soundness and rarity; but because it can only be cut by laser, there are very few large polished carbonados, and stones with a known history are collector’s pieces. A laboratory report matters so that it is not confused with heat-treated single-crystal black diamonds or with black imitations.