Black Diamonds and Carbonados: A Reflective Overview

Robin F. Hansen, BSc (Hons) FGA and
Mineral and Planetary Sciences Division, Earth Sciences Department, Natural History Museum, London, United Kingdom Léonie J. Rennie BSc MEd PhD FGAA
School of Education, Curtin University, GPO Box U1987, Perth, Western Australia 6845

Acknowledgements

The authors gratefully acknowledge the generous cooperation of Sotheby’s and Royal Coster Diamonds for allowing access to images from their collections; to Brendan Cunningham of Salt and Pepper Diamonds for use of an image of a ring. The authors are also indebted to Dr A.J.A (Bram) Janse for his comments on an earlier draft of the manuscript, and to Francine Payette, for assistance in translating the French text by Moissan.

Abstract: Inspired by the loan of a beautiful black diamond to the Natural History Museum, London, and the frequent lack of detailed information about black diamonds in gemmological texts, this article reviews the nature, colour, and origin of black diamonds and carbonados. The article concludes with descriptions of two famous black diamonds: the Black Orlov and the Black Amsterdam, and two carbonados, the Sergio and the Enigma.

Abstract

Inspired by the loan of a beautiful black diamond to the Natural History Museum, London, and the frequent lack of detailed information about black diamonds in gemmological texts, this article reviews the nature, colour, and origin of black diamonds and carbonados. The article concludes with descriptions of two famous black diamonds: the Black Orlov and the Black Amsterdam, and two carbonados, the Sergio and the Enigma.

Introduction

A very large and beautiful black diamond was recently displayed in “The Vault”, the space in which the Natural History Museum (NHM), London, displays some of its most stunning gem specimens. Named by its owner after his daughter, the “Anastasia Diamond” was loaned temporarily to the Museum for display. Measuring 37.8mm by 25.3mm, the stone weighs 93ct, and is certified as a natural diamond of natural black colour. The stone has been in the lender’s family since the late 1800s and was acquired in Goa, then a famous gem cutting centre in India.

The specimen has been beautifully cut in a pear shape and set in an Arabian-style pendant in white gold; a clever choice that contrasts with the remarkably high lustre of the black diamond. Atop the front of the setting are three stars set with yellow diamonds below a crescent moon highlighted by colourless diamonds. The crescent moon is mirrored by another crescent of colourless diamonds set along the right-hand edge of the black diamond. The back of this setting, made visible to the viewer by a carefully placed mirror, depicts the constellation of Ursa Major, again set with yellow diamonds. In all, the setting contains 26 yellow and 41 colourless diamonds. This is a spectacular piece (Figures 1 and 2).

The loan of this stunning piece prompted the authors to reflect on the nature of black diamonds more generally and, until recently, their frequent absence in gemmological texts. The comprehensive and well-illustrated volume, entitled Diamonds (Maillard, 1980), for example, mentions black diamonds in only three places. In the introductory chapter, Lenzen and Delveaux (1980) wrote that in India black diamonds were dedicated to Yama, the god of death (p.15), and that a “black diamond referred to by Pliny was probably siderite” (p.22). The third mention was in the chapter on clarity and colour (Gübelin, 1980, p.229) where a small photo of a brilliant cut black diamond was pictured but not referred to in the text. Other books focused on diamonds published around this time, such as Blakey (1977) and Bruton (1978), mention famous black stones, such as the Black Amsterdam and the Black Orlov (see later for descriptions of these remarkable specimens), but do not elaborate on black diamonds per se.

Although black diamonds have long been known for their industrial uses, their popularity for jewellery has increased over the last few decades, stimulated, at least in part, by publicity surrounding black diamond engagement rings given and received by various celebrities. A relatively recent and well-referenced example occurred in the movie Sex in the City 2, in which Mr Big gave Carrie a black diamond engagement ring. A quick search on the internet will afford readers the opportunity to purchase an identical beauty!

An internet search for “black diamond” brings up many references to their reputed mystical powers, rather than information about their gemmological properties. There is still much more to be known about their nature, colour, and origin, and in this short article we explore these aspects. We conclude with descriptions of four famous black diamonds.

What are Black Diamonds?

The GIA Diamond Dictionary (Liddicoat, 1993) provided a concise description of “black diamond”, and we reproduce it here in full:

(i) Very dark gray, blue, brown, or green diamond, or one which appears black or opaque due to numerous submicroscopic inclusions (often graphite) which block all, or nearly all, transmission of light. Black diamonds may have transparent colorless to grayish areas alternating with opaque areas. Color can be modified by blue, brown, gray, green, olive, or purple. Such stones may show mottled strong blue fluorescence under long wave ultraviolet radiation, chalky greenish fluorescence under short wave ultraviolet. Black diamonds typically show no absorption spectrum. When the color is caused by inclusions, the surface is often pitted and fissured and has an almost metallic appearance; they are very difficult to polish.

(ii) Carbonado

(iii) Misnomer for faceted hematite or anthracite coal (p.23).

We will ignore the third part of this description – neither hematite nor anthracite can be confused gemmologically with diamond – and devote our attention to the other parts, in turn. Before doing so, we note that the terms “black diamond” and “carbonado” are often used interchangeably, but they refer to different substances. Both may be described as polycrystalline because they comprise many, often submicroscopic, interconnecting diamond crystals, but techniques developed over the last half century have enabled greater scrutiny, revealing that their physical and chemical properties differ sufficiently to indicate two distinct types of diamond. These differences will become clear in the descriptions below, but a significant geological difference is that confirmed sources of carbonado are located only in the Bahia district of Brazil and the Central African Republic in alluvial deposits and, unlike other polycrystalline diamonds, carbonados have never been traced back to originating mantle rocks.

The nature of black diamonds

The majority of all diamonds found are not suitable for use as gemstones and are used in industry. Industrial diamonds are mostly polycrystalline, usually dark coloured, included, and opaque. They are generally called boart (also boort or bort), a name which Harlow (1998) suggested may have come from a Middle-French term meaning “bastard”. Being polycrystalline means there is some porosity, so the specific gravity is variable but generally lower than that of monocrystalline diamond. The variability depends on the degree of porosity and the nature of the inclusions. The lowest quality boart is suitable only for crushing and use as industrial abrasives (Wilks and Wilks, 1991). Better quality boart, in which the component diamond crystals interlock very closely, has greater hardness and the material has extensive use in industry, in drilling bits, for example, where monocrystalline diamonds would fail due to their cleavage, or, if heavily included, disintegrate due to frictional heat.

Figure 3 shows two quite large specimens of rough, industrial diamonds from the collection of the NHM. Both are the black cryptocrystalline variety of boart, but both also show hints of crystal faces and shape accompanying normal crystal development. The specimen on the left (BM.1985,MI32223) is 16mm in diameter, and weighs 7.151g (35.75ct). On the right is a slightly larger specimen (BM.1985,MI32222), 18mm in diameter, weighing 8.701g (43.50ct). Both are from South Africa, mostly likely the Premier mine.

Particular forms of natural polycrystalline diamond are recognised. Ballas, or spherical boart, is “made up of spherical masses of minute, intergrown diamond crystals arranged concentrically” (Liddicoat, 1993, p.18). It is highly compacted, with a grey colour (Wilks and Wilks, 1991). An example of ballas, with beautifully spherical nature, is displayed in Figure 4. This specimen (BM.41401) is from Brazil and was presented to the NHM by G. F. Coster in 1868 (see Note 1). It weighs 0.495g (2.48ct) and is 6.5mm in diameter. The rarer framesite is generally black, composed of aggregated clusters of submicroscopic diamonds and is one of the hardest diamonds. Found primarily in South Africa and Botswana, framesite was named after P. Ross Frames, Chairman of De Beers and Premier companies in the 1920s (Heaney, et al., 2005). An even rarer kind of boart is stewartite. It has a lumpy, cinder-like structure and dull sheen; it is magnetic due to a high level of magnetite impurities. It was named for James Stewart, a former manager of the Kimberley Treatment Plant (Liddicoat, 1993).

Industrial diamonds that are monocrystalline, but too flawed or included for use as gems, can be used when the optical, chemical or physical properties of diamonds are required. For example, in the early 1900s Thomas Edison employed the hardness of industrial diamonds in the styli for his celluloid cylinder and diamond disc phonograph records, carefully polished so that the point fitted snugly in the record groove (Figure 5).

Both monocrystalline and polycrystalline diamonds maybe used as a dark coloured gemstone when of sufficient coherence to allow cutting, however, as they are always heavily included, flawed, and invariably fractured, their cutting and polishing is a challenge requiring great care. The partially faceted black diamond shown in Figure 6 demonstrates this problem. Specimen BM.1914,1373 measures 10.80mm x 11.03mm x 7.2mm and weighs 1.377g (6.885ct). It resembles a modified brilliant cut, has eight pavilion facets and then a chequerboard appearance as the table is the same size as the other facets on the crown. It is poorly faceted, unpolished around the girdle, and the label indicates that it was only partly cut and polished due to the extreme hardness. The diamond was found in Kimberley, South Africa around 1884 and donated to the NHM in 1914. A type-written note relating to its provenance and attesting to its hardness is included as Figure 7. The note is edited and signed by G.F.H.S (George Frederick Herbert Smith; see Note 2).

Colour of black diamonds

Naturally “black” diamonds are not always black. Some cut gemstones with a black face-up colour may have a dark brown or dark green colour, more easily seen from the side, or by shining a very strong light through the stone. Black diamonds owe their dark colour to several sources: inclusions, staining, fractures, and a polycrystalline nature. Numerous submicroscopic inclusions are typical; usually of graphite which may be needle-like or tabular, but also sulphides and some iron oxide-bearing inclusions. Some diamonds contain “clouds” of micron- to nano-sized inclusions that are usually graphite. Black diamonds may also be irradiated from natural sources that can cause a brown or greenish colour at the surface and some graphitisation in internal fractures. 

The grain boundaries in polycrystalline diamonds, their inclusions and fractures can all impede the transmission of light making the diamond appear opaque, even when transparent or translucent areas are present. When the pattern of inclusions and/or fractures gives a scattered whitish and dark appearance, the diamonds may be described, and sold, as “salt-and-pepper” diamonds. Figure 8 shows an example of a ring set with three salt-and-pepper diamonds. It features a 1.65ct hex-cut diamond framed by two oval-cut yellow diamonds.

Intensive research into the cause of colour in black diamonds is relatively recent. A study by Smit et al. (2018) examined 40 natural black, untreated diamonds from alluvial deposits in eastern Zimbabwe. These stones were brilliant cut and described as Fancy Dark Brown to Fancy Black. The reporting paper is well-illustrated and shows the microstructure of the diamonds very clearly. By using a range of spectroscopy techniques, Smit et al. found that their colour was due to a combination of graphite micro-inclusions, graphite needles, and dark brown radiation stains that occurred along internal fractures that reached the surface. The authors attributed the radiation stains to natural radioactive fluids penetrating the diamond along the fractures, and concluded the radiation to be natural, arising from radioactive minerals in nearby conglomerates. They also considered that relatively high temperature metamorphism had resulted in a brown, rather than the green, radiation stain caused by radioactive elements. Four of the 40 diamonds examined by Smit et al. had their colour caused by clouds of micron-sized inclusions of graphite; only one of these had some radiation staining.

Ardon (2013) examined a brilliant-cut black diamond submitted to the GIA laboratory. He determined that although there were some small black inclusions, its colour was due almost entirely to dark brown radiation staining of its extensive fractures. Ardon believed it to be unusual that natural radiation rather than inclusions was the main source of colour. More recently, Eaton-Magaña et al. (2019) published an analysis of colour in those diamonds referred to as Fancy White and Fancy Black. (As an aside, we note that the milky colour of white diamonds is due to nano-inclusions and disruptions in the crystal lattice.) The photographs in Figure 5 of the article by Eaton-Magaña et al. (2019, p.324) present a range of faceted Fancy Black diamonds, showing that inclusions may be in bands, or masses of various size, sometimes with staining of iron oxide, and variously shaped inclusions of graphite. A random selection of 1000 Fancy Black diamonds was examined to identify the major cause of colour. Eaton- Magaña et al. reported that about 43% contained clouds of nano-inclusions and radiation staining was visible in about half of those. Crystal inclusions were responsible for the colour in 46% of the stones and about a fifth of these showed radiation stains, generally brown. Around 10% of stones owed their colour to inclusions of both crystals and clouds. Only 1% of the 1000 faceted stones had colour due to absorbing effects or polycrystalline growth, and these stones tended to be deep violet or purple. About half of those diamonds with clouds generally fluoresced weakly with dull yellow; those with crystals mostly did not fluoresce.

Treatment of black diamonds

Black diamonds are generally referred to as either natural or treated. Fritsch (1998) explained how the increasing popularity of black diamonds for use in jewellery has justified treatment to improve the appearance of poor quality, low clarity stones. Overton and Shigley (2008) provided a comprehensive coverage of the treatment of all kinds of diamond and explained these processes in more detail. Today, most black diamonds used in jewellery have their colour caused, or enhanced, by various treatments. Eaton-Magaña et al. (2019) reported that over one third of the black diamonds examined by the GIA Laboratory have been treated. 

Stones referred to as treated have been heat-treated, generally at low pressure and high temperature (LPHT) to graphitise the numerous fractures and create the black appearance. Less fractured stones can survive high pressure and high temperature (HPHT) treatment. Radiation from artificial sources can also darken the colour. Kitawaki (2007) described common treatment techniques as electron beam irradiation with annealing (heat treatment), and high temperature heat treatment in a reducing atmosphere. The numerous inclusions in black diamonds usually result in a pitted surface and poor lustre when polished, but low-grade or low clarity diamonds treated by irradiation can have improved colour, and also a superior lustre on the polished surfaces.

Efforts to distinguish the kinds of treatments have become increasingly successful, but they often rely on techniques not usually accessible in every laboratory. Kitawaki (2007) pointed out the need for disclosure of all such treatments when the diamonds are traded, and disclosure should be routine.

Grading of black diamonds

Whilst “black” diamonds are not always black – they can range through greys, greens and browns, even with blue or purplish overtones –they are very dark and almost always opaque so they cannot be graded using the traditional 4Cs method of diamond grading. Their colour can be evaluated on GIA’s colour grading system, but clarity cannot be evaluated. Cut and carat weight are still relevant in assessing the stone. The GIA does not issue grading reports for black diamonds, but instead issues a “Colored Diamond Identification and Origin Report”, in which black diamonds are described as Fancy Black and their colour is noted as either natural or treated (GIA, 2021).

Carbonado

The first black pebbles to be recognised as diamonds were found in alluvial deposits along with “normal”, crystalline diamonds in Bahai, Brazil, in the mid-19th century. They soon became known more broadly; a Mr Mayer displayed several from this locality on December 1, 1845, at the Literary and Philosophical Society of Liverpool (Proceedings, 1845, p.16). They were described as “of dark colour, deficient in lustre, and of little value, except in the manufacture of watches”. (At this time, due to their hardness, small industrial diamonds, rubies and sapphires were used in some time pieces as fulcrums for moving parts because they did not wear out.) A year later, Mr Mayer presented another brown diamond “of considerable magnitude”, from the same location, and stated that “he had found it much harder than the common transparent diamond of commerce, so much so, that no means he had hitherto tried of cutting it had succeeded” (Proceedings, 1846, p.33). It is likely that this specimen was that exhibited by Mr Joseph Mayer, of Liverpool, at the Great Exhibition of 1851. A guidebook for the exhibition published by the Illustrated London News referred to “the black diamond (not a coal, though a carbon), exhibited by Mr Joseph Mayer, which may be regarded as a very great curiosity. It weighs 350 carats and is so extremely hard that it has resisted every attempt that has been made to polish it” (Cantor, 2016, p.163). Amongst others, Liddicoat (1993, p.23) referred to this diamond as the Black Diamond of Bahia, noting that its current whereabouts is unknown.

These historical references identify two essential characteristics of carbonado: dark in colour and incredibly hard. Reputedly, these very dark diamonds received the name “carbonado” from the Portuguese word for “burned” or carbonised, perhaps because they look like coal and the exterior surface of rough carbonado often has a glossy patina. That they are indeed made of carbon was determined by chemical means. Dufrenoy (1849) described in detail some carbonado specimens acquired by the French École des Mines, noting the porosity and its effect on specific gravity. He also reported on experiments in which specimens burned in pure oxygen gas returned over 99% carbon.

The description of carbonado in the Diamond Dictionary (Liddicoat, 1993) is
“Carbonado, polycrystalline aggregate of minute diamond crystals with a granular-to-compact structure; it may be black, brown, or dark grey. Used for industrial purposes, carbonado is the toughest form of diamond” (p.37). The effectiveness of carbonados as abrasives quickly increased their value and from the start of the second industrial revolution, beginning around 1880, Brazil held a half-century monopoly for the production of carbonados (Herold and Rines, 2011). This revolution drove the use of industrial diamonds for working with steel, drilling for oil and minerals, and building public infrastructure. Early in the 20th Century, the carbonado type of polycrystalline diamond was found in the Central African Republic, and here and the Bahia area of Brazil remain the two main sources of carbonado.

Heaney et al. (2005, p.86) described this polycrystalline form of diamond as having “euhedral grains (typically up to 200μm) set in a microcrystalline matrix (<0.5–20μm)” with a high void space of around 10%. Estimation of void space in 240 carbonados by Haggerty (2017) ranged from around 6% to 13%. Framesite and ballas, the natural polycrystalline diamonds found in kimberlites, have a similar texture but with larger grains and very little void space (Heaney et al., 2005; Wilks and Wilks, 1991). The voids in these polycrystalline diamonds tend to be irregularly polyhedral, whereas the pores in carbonado are either spherical or oblate (Haggerty, 2014). Further distinguishing them from other black diamonds, carbonados in the rough have a patinated, glassy appearance (see photos in Haggerty, 2014, 2017). The toughness and tenacity of carbonado are legendary; they stem “from the random orientation of microdiamonds, are clearly superior to monocrystalline gem diamond, to the point that carbonado can only be cut by lasers” (Haggerty, 2017 p.173).

Some examples of carbonado from the NHM collection are shown in Figures 9 and 10. Figure 9 displays a large carbonado (BM.23442) from Brazil that was purchased from a Mr Sowerby in 1849. The rounded pores, striations, and glossy patina typical of carbonados, are obvious. This fine specimen measures 28mm x 22mm x 14mm and weighs 12.944g (64.71ct). Figure 10 is specimen BM.41402, which includes two carbonados, also from Brazil, that were presented by G. F. Coster in 1868, together with the specimen of ballas (BM.41401) described above and illustrated in Figure 4. The specimen on the left of Figure 10 is 0.088g (0.44ct) and 4mm wide; the specimen on the right is 0.101g (0.50ct) and 5mm wide.

The colour of carbonado is usually black, grey or brown. It is so hard that it is generally cut only by laser and very few have been cut as gemstones. Eaton-Magaña et al. (2019, p.325) pointed out that the polycrystalline structure results in varying directions for effective polishing and achieving a flat and reflective surface on facets is very difficult. Further, the generally higher porosity of carbonado compared with other polycrystalline diamonds affects the lustre.

Origin of carbonado

The first theory of origin for carbonados was similar to that of other diamonds: formed deep in the Earth’s crust in conditions of very high pressure and temperature then brought to the surface by volcanic activity. However, carbonados are invariably found in alluvial deposits and have never been traced back to a mantle rock source like other diamonds. Further, the mineralogy of their inclusions is inconsistent with this theory. Half a century ago, Trueb and de Wys (1969) examined specimens of carbonado from Bahia under reflected light, finding that the stones were very porous, accounting for a lower density than pure diamond. Pores were irregular, mostly elongated and often contained yellowish, white, or pink crystalline inclusions that were mainly silicates, predominantly orthoclase. These inclusions were epigenetic, unlike the inclusions in black diamonds which tend to be syngenetic. An idea that they were formed by radiation was discounted because natural irradiation would be insufficient to produce carbonados as large as those found.

By the late 20th Century, extra-terrestrial origins were becoming favoured, with small diamonds confirmed in some chondritic meteorites (Huss, 1990). Perhaps the strongest early support of this theory comes from a paper published in 2006 by Garai et al. who first removed the silicon contamination from carbonado samples obtained from both Central African Republic and Brazil, then subjected the specimens to examination by infrared Fourier Transform Infrared (FTIR) absorption spectroscopy. They compared the results with the FTIR spectra of diamonds from meteorites and interstellar dust and based on the close resemblance, and the other mineralogical features of the specimens, concluded that their measurements were “consistent with an origin of carbonado diamonds in an interstellar environment. The implication is that carbonado diamond is of asteroidal proportions” (Garai et al. 2006, p.653). Given that Brazil and Central African Republic were once joined side by side as part of a super continent, the collision of rocks from space could explain why these areas are the main source of carbonado diamonds. The carbonados from both Bahai and Central African Republic are at least 3.8 Ga old, and similar in age to “the terminal cataclysmic event (4.1- 3.8 Ga) in the solar system that resulted in intense meteoritic bombardment” (Haggerty, 2014, p.67), whereas the oldest diamonds from kimberlites and lamproites are 3.5–3.3 Ga found from the Ekati and Diavik mines in Canada (Shigley, et al., 2016).

The ambiguity of the origin of carbonado became known as “the carbonado conundrum”, and in an attempt to make sense of this, McCall (2009) reviewed a range of literature relating to theories of the origin of carbonado. He concluded that the evidence was “highly contradictory” but found it “very difficult to envisage…any source …other than the deep Earth mantle” (2009, p.90). McCall did not cite Haggerty in his review, although by 1999 Haggerty had begun to link carbonados with extra-terrestrial events (Haggerty, 1999). Cartigny (2010) described diamonds (not carbonados) from komatiite in French Guinea as having very similar geochemical properties to carbonado (notably the carbon and nitrogen isotopic concentrations), thus speculating that carbonados could be formed in mantle conditions. Since then, based on a sample of about 800 carbonados, Haggerty (2014) has presented a very thorough analysis of their physical and chemical properties, and a detailed examination of theories of their origin. Here, and in a review of this work for Gems & Gemology, Haggerty (2017) promoted an extra-terrestrial origin, discounting impact and deep mantle theories of origin. He concluded that “The extra-terrestrial model, although conceptual and supported by astrophysical data, will only be vindicated by the discovery of carbonado in the asteroid belt by remote sensing, or by an observed diamond meteorite fall that is dark in colour, porous, and patinaed” (p.177).

Distinguishing Between Carbonado and Black Diamonds

To summarise the above discussion, Table 1 presents an overview of the properties of carbonado and other black diamonds, both monocrystalline and polycrystalline. The table is based primarily on data from Haggerty (2014), Heaney et al. (2005) and Trueb and de Wys (1969).

Two Famous Black Diamonds

The Black Orlov Diamond

Probably the most famous black diamond is known as the Black Orlov (or Orloff), a lovely oval, cushion cut stone, set in a platinum, leaf-like surround with colourless diamonds (Figure 11). Now a 67.49ct stone, it was reportedly cut from a rough stone of 195ct. The brooch has 108 diamonds and is suspended from a 40cm platinum necklace, encrusted with 124 marquise and round cut diamonds. The Black Orlov is actually a very dark gunmetal colour, but gemmologically, it is called a Fancy Black. A report dated 11 February 1998 from the GIA stated the diamond to be Fancy Black, natural colour (Christie’s, 2022a).

Its early history is shrouded in rumour and myth. Reputedly, it was at one time owned by a Russian Princess Nadia Vyegin-Orlov, from whence it gained its name. It is also said to have been stolen from an Indian shrine in the 19th Century so is additionally known as the Eye of Brahma, and therefore cursed. A Smithsonian publication (2016, p.271) provides a somewhat tongue-in-cheek timeline, labelled pre-1800s to 2006, for the Black Orlov, including some of the myths. It seems clear that most of the early history is fabricated, and even its alleged place of origin as India is unconfirmed. Some of the mystery may arise from confusion between the Black Orlov and the magnificent 189.62ct faint bluish-white stone, also called the Orlov (or Orloff). This Orlov diamond did originate in India and is sometimes known as The Great Mughal diamond. It was set in the sceptre of Catherine the Great of Russia, where it remains as part of the Russian Imperial Sceptre.

The history of the Black Orlov has been researched by Zwart (2018), in parallel with the bluish-white Orlov. Zwart was able to discount as mythology much of the early history of the Black Orlov and concluded that its first authenticated owner was Charles Winson, a New York gem dealer, who had it split into three pieces (reputedly to destroy its curse) and set. He exhibited it in many places, including the American Museum of Natural History in 1951, the Texas State Fair in 1964, and in Johannesburg in 1967 (Balfour, 1997).

The Black Orlov was sold several times at auction and when owned by J. Dennis Petimezas of Pennsylvania, it was exhibited in London in 2005 as part of the Diamond Exhibition at the NHM. (This exhibition was attended by the authors who, understandably, were overwhelmed by the dazzling display.) Unfortunately, the exhibition closed early on advice from the Metropolitan Police that there were credible threats of theft (Prudames, 2005). Shortly after, the Black Orlov was sold at auction by Christie’s, on October 11, 2006, for USD352,000 (Christie’s, 2022a), and its current owner is anonymous.

The Amsterdam Black Diamond

Whilst not as large as some other famous black diamonds, the Amsterdam Diamond is unusual because it is a monocrystalline black diamond of remarkable opacity. The 55.85ct rough was purchased from South Africa as boart by Amsterdam diamond merchants D. Drukker & Sons of Amsterdam in 1972 (Blakey, 1977), when the average price for boart was around US$6.00 per carat (Balfour, 1997). Recognised as a significant specimen, it was cut in-house in a modified pear-shape with 145 facets, losing only about 40% of its weight to finish at 33.74ct. It was set as pendant surrounded by 15 cushion-cut diamonds (see Figure 12). Its first public display was in 1973 at the Great Diamond Exhibition that celebrated the 700th anniversary of the city of Amsterdam. The Diamond Museum of Amsterdam (n.d.) reported that it was the showpiece of the Exhibition, and the diamond was christened to bear the city’s name.

The Amsterdam remained in the hands of Drukker & Sons and was exhibited in several places for charitable purposes (Balfour, 1997). In 2001 it came to Christie’s for auction as an unset stone in a blue leather fitted case (Christie’s, 2022b). Amongst its provenance documents were three certifications of its status. It was certified by the GIA in 1999 as a Fancy Black, natural colour; by the Gübelin Gemmological Laboratory in 1999 as black, natural colour; and the Gem Tech Lab, Geneva, in 2001, as black, natural colouration. This last laboratory report indicated that the diamond:

owes its colour to the presence of small graphite grains mainly located in the feathers, and to diffusion mechanisms similar to those observed in storm clouds. … The gem was cut in 1972 by the firm D. Drukker & Sons of Amsterdam. The cutting has been executed very carefully, without excessive heat, and has not generated pyrolitic graphites that may have intensified the colour of the stone. This gem can be considered as representative of the best quality in black diamond (Christie’s, 2022b).

The Amsterdam Black diamond was sold to an anonymous buyer on November 14, 2001, for USD352,000.

Two Famous Carbonados

The Sergio Carbonado

In 1895, Henri Moissan (after whom the mineral Moissanite was named) reported in the French journal, La Nature, on a specimen of carbonado “found on July 15, 1895, in a diamond field in the province of Bahia, in Brazil, by a miner by the name of Sergis Borges de Carvaloo” (Moissan, 1895, p.304). At that time, carbonados were frequently referred to as “carbons”, and Moissan went on to describe it as follows:

This new carbon, rounded in shape, is very dark black, and its surface varies from granular to smooth. The granular part, examined with a magnifying glass or under a microscope with a low magnification, has the appearance of a material which has released gases being still in the pasty [or viscous] state. It looks a lot like the surface of the microscopic carbon grains we got in our silver and iron pellets compressed by sudden cooling in water. The colour is also the same. This carbon is porous; since it was taken out of the ground, that is to say two months ago, it has lost about 19 grams in weight; at the time it was found, it weighed 3167 carats. It is the finest sample which has been encountered so far in the diamond fields of Brazil (Moissan 1895, p.304, authors’ translation from French).

The Sergio remains the largest diamond ever recorded, about 60ct heavier than the South African Cullinan diamond (discovered in 1905), which was 3,106ct in the rough. Moissan’s report included an engraving of this carbonado, which is reproduced in Figure 13.

An additional report of the Sergio is in the catalogue for the Brazilian display at the 1904 Louisiana Purchase Exposition at St Louis. Labelled the “Famous Carbon (Black Diamond)” (Brazil, 1904, p.38), it was described as “the largest carbonate [sic] stone ever seen … weighed 3,150ct … taken to Paris it was broken into small pieces to be used in industry” (p.38). In 1906, Furniss, formerly the American Consul at Bahia, reported that this carbonado:

was purchased from the miner for $16,000 and was finally exported to London, where it sold for $31,145, having lost about 50 carats meantime in drying out. In London it was broken into pieces suitable for drills and these pieces sold for about $40,000, while at the present price of carbon they would be worth about $158,000 (Furniss, 1906, p.6).

Furniss (1906) included a photograph of the Sergio (shown here as Figure 14), which is identical to the photograph in the Catalogue for the 1904 Louisiana Purchase Exposition (Brazil, 1904, p.19). Judging by its outline, this photographic image appears to be the other side of the same stone that appeared in the Moissan (1895) engraving (see Figure 13). This photograph appears in many places, including Herold and Rines (2011, p.25), in their historical account of the importance to the economy of Brazilian carbonados, who attributed their image of this stone to Furniss (1906), and Haggerty (2014, p.54), who used the same image, but rotated 90˚ anticlockwise.

Like many famous stones, there are irregularities in reports of its weight, the date it was found and what happened to it. Its weight is usually given as 3,167ct (see for example, in the list of large Brazilian diamonds by Svisero et al., 2017, p.6), although clearly it must have varied given that both Moissan (1895) and Furniss (1906) refer to its drying out. The date of finding is often given as 1905, for example by Haggerty (2014, 2017), which must be incorrect given that the image of this stone appeared in the 1895 article by Moissan. Further, given that there seems to be only one image of the Sergio to be found, it is very likely that it was indeed broken up prior to 1904, the date given in the Catalogue for the Louisiana Purchase Exposition, and is no longer in existence.

The Enigma Diamond

As we were writing this article, a new black diamond made headlines around the world. The Enigma is the largest known faceted carbonado and came to Sotheby’s in London for auction in February 2022. The Enigma diamond was described in the Guinness World Records in 2005 as the largest cut diamond, noting that:

It weighs 555.55 carats and was polished into 55 facets over several years and completed in June 2004. The repetitive use of the number five is culturally significant in the Islamic world, and was inspired by Ran Gorenstein (Belgium), who also commissioned this creation (Guinness World Records, 2022).

The description accompanying the auction details (Sotheby’s, 2022) include the dimensions of 54.13mm x 44.86mm x 32.25mm, and this impressive specimen is shown in Figure 15. The GIA report of June 22, 2004 stated that this is the largest Fancy Black diamond they had reported on, to that date, and add that it is an unusual form of diamond known as carbonado (Sotheby’s, 2022). A Gübelin Laboratory report of August 2004 describes the diamond as natural colour, Fancy Black, and a November 2021 report from the same lab confirms the identity of the diamond. This latter point is significant, because a replica of the Enigma was exhibited at the Shanghai Expo in 2010 for a period of six months, enabling over 71 million people to view it (Sotheby’s, 2022).

That the Enigma is indeed a carbonado is remarkable, given how difficult they are to cut and polish. Sotheby’s (2022) stated that the consignor bought the carbonado in the 1990s and in rough form the weight would have exceeded 800ct. Cutting and attaining the high degree of polish took over three years. The resultant 555.55ct specimen, with 55 facets was inspired by Hamsa, the Middle Eastern palm symbol of protection, power, blessings, and strength.

It seems likely that the Enigma is the largest diamond that has been cut and polished. Balfour (1997) described the Golden Jubilee diamond as the largest diamond to have been cut and polished to that date. It weighed 755.50ct in the rough and after faceting weighed 545.65ct in a modified brilliant cut. This is just 10ct less than the Enigma, but the two diamonds are quite different stones and cannot be compared on any basis other than weight.

The theory that carbonados have an extraterrestrial origin was an aspect picked up by much of the media coverage that accompanied the launching of the sale, with news stories of the Enigma’s uniqueness querying its origin. For example, the Sydney Morning Herald headlined “Out of this world: mysterious black ‘Enigma’ diamond up for auction” (Ross, 2022); others referred to it as a “mystery” diamond, as suggested by its name.

The auction at Sotheby’s realised USD4.3m and the buyer was not named.

Conclusion

Black diamonds were traditionally used for industrial purposes but in recent times have become more commercially valuable in jewellery. Those specimens large enough, and sufficiently coherent to be faceted, create beautiful, fascinating gemstones. Natural black diamonds from kimberlite origin are different to carbonados, and there are famously large examples of each. The above stories reveal that large diamonds generate a range of mystery, myths and misinformation. One wonders whether this new, and larger, Anastasia diamond that was loaned to the NHM will eventually gather as much folklore as the Black Orlov as time goes by! It certainly gathered admiration when in distinguished surroundings, not far from the famed Aurora Collection of coloured diamonds, also on loan to the NHM and displayed in The Vault.

Notes

Note 1. Royal Coster Diamonds is now the world’s oldest diamond polishing factory. It was established in Amsterdam in 1840 by diamond cutter Moses Elias Coster. In 1852, at the request of Queen Victoria, two of Coster’s diamond polishers were given the task of repolishing the Koh-I-Noor diamond, which is one of the significant diamonds in the British Crown Jewels. Moses Elias Coster had 11 children, one of whom was Guillaume Frederick Isidore Coster (also known as Willem), and he seems to be the G.F. Coster who made the presentation of several specimens of black diamonds to the NHM in 1868. In 2016, Coster Diamonds became Royal Coster, when King Willem-Alexander of the Netherlands granted the Honorary title Royal. This title may be bestowed on an organization that is leading in its field of expertise, is of national importance and has been in existence for at least 100 years.
More history of Royal Coster is available from its website
https://www.costerdiamonds.com/costertimeline.

Note 2. George Frederick Herbert Smith joined the Mineralogy department of the British Museum in 1897 and on his arrival, took over the work of selecting specimens for purchase or exchange. He specialised in the study of the physical properties and improvement of optical instruments for this work (Smith, 1950). In 1907, he produced the first efficient jeweller’s refractometer with a scale calibrated in refractive indices, and wrote a significant text entitled “Gemstones”, first published in 1912 (Webster, 1953), later revised by F. Coles Phillips, rewritten and republished many times. All gemmologists are familiar with the message in the first words of Herbert Smith’s introductory chapter in the first edition of his book: “Beauty, durability and rarity: such are the three cardinal virtues of a perfect gemstone” (Herbert Smith, 1912, p.1).

References

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Balfour, I., 1997. Famous diamonds, 3rd ed. London: Christie, Manson and Woods.

Blakey, G., 1977. The diamonds. New York and London: Paddington Press.

Brazil, Commissão, Louisiana Purchase Exposição, 1904, and F. M. De Souza Aguiar. Brazil at the Louisiana purchase exposition. St. Louis, Saml. F. Myerson Ptg. Co. [online] Available at: <https://lccn.loc.gov/18006587> [Accessed 16 January 2022].

Bruton, E., 1978. Diamonds, 2nd ed. Radnor, PA: Clifton Book Company.

Cantor, G. ed., 2016. The Great Exhibition: a documentary history, Volume 3. Abingdon: Routledge.

Cartigny, P., 2010. Mantle-related carbonados? Geochemical insights from diamonds from the Dachine komatiite (French Guiana). Earth and Planetary Science Letters, 296, pp.329-339.

Christie’s, 2022a. A rare coloured diamond pendant necklace. [online] Available at: <https://www.christies.com/en/lot/lot-4785824> [Accessed 7 February 2022].

Christie’s, 2022b. The Amsterdam Diamond. [online] Available at: <https://www.christies.com/en/lot/lot-3816656> [Accessed 7 February 2022].

Diamond Museum of Amsterdam. n.d. Timeline. [online] Available at: <https://www.diamondmuseum.com/exhibits-themes/timeline/timeline/> [Accessed 6 February 2022].

Dufrenoy, P., 1849. A report on the discovery of carbonado diamond. Philosophical Magazine, 34(230), pp.397-398. [online] Available at:
<https://babel.hathitrust.org/cgi/ssd?id=uva.x002266060;page=ssd;view=plaintext;seq=412;num=398> [Accessed 2 November 2021].