Black Diamond, also known as carbonado, is a polycrystalline variety of diamond and is extremely rare - the rarest of all diamond types: Since 1900, around 600 tons of "regular" diamonds have been mined  whereas only around 2.5 tons of black diamonds have ever been discovered. 
Black diamond is made up of a porous aggregate (mixture) of minute black crystals.  Not only is black diamond an extremely rare gemstone, it can quite rightly claim to be the world's most mysterious gemstone - as there is still so much about it that is not known for certain.
The 312.24 carat Spirit of de Grisogono black diamond
in a ring setting
Black Diamond Origins
Black diamonds have an air of mystery around them which is strangely appropriate to their shadowy color: Their true origin is still unknown to geologists. Several interesting theories exist in this regard; including the hypothesis that a supernova occurring several billion years ago ejected a mass of material - of which some fell to earth in the form of a meteor. The latter event is believed to have occurred prior to the separation of the continents, in a region which later split into Brazil and Africa. The majority of black diamonds have been found either in Brazil or in Africa (principally the Central African Republic). 
Other theories for the origin of black diamonds include: Formation under high pressure inside the Earth (as with other diamonds); metamorphism induced by meteorite impact; and formation caused by fission of uranium and thorium. Black diamonds also do not contain traces of "mantle minerals" that are commonly included in clear diamonds , yet contain unusual concentrations of "highly reduced metals and metal alloys". 
Although many sources state that the only locations in which black diamonds have been found are Brazil and the Central African Republic, the uncut stone in the image (below right) is clearly stated by its photographer Rob Lavinsky, a noted authority on minerals, to have originated in Sierra Leone, a country in the far west of Africa. Other sources state that black diamonds are "almost exclusively" found in Brazil and Africa but that they are exclusively found in alluvial deposits - placers created by running water - which indicates they may once have been at the earth's surface.  Black diamonds are not associated with kimberlites or lamproites, which indicates that a different process led to their formation,  however some have been found in placers in which transparent diamonds also occurred.
Recently, black diamonds were discovered in Yakutia, Siberia. A Gems & Gemology article from 2003 reports on scientific analysis on these stones, which were found to have a different chemical composition to the African / Brazilian material.  Owing to the mineral differences in the polycrystalline diamonds from Yakutia, the name yakutite has been suggested for these stones; however they are still considered carbonados, with minor differences to the African / Brazilian kind - and their origin is still described as a "conundrum". 
Uncut Black Diamond
23.95 cts : 15 x 15 x 15 mm. From Sierra Leone (in West Africa)
Photo by Rob Lavinsky, iRocks.com -
image lic. under CC-BY-SA-3.0
Who would guess that this is an ultra-rare precious diamond?
Overall, black carbonado diamonds exhibit several qualities which are incompatible with formation deep inside the Earth, and this has led geology professor Stephen E. Haggerty, publishing in the Astrophysical Journal, to the next logical conclusion: That the black diamond is of extraterrestrial origin.  Research using infrared rays (2007) has shown black diamonds to have a chemical spectrum which includes hydrogen, in an amount which suggests that they may have originated inside a star. 
Nonetheless, the black diamond's origin is still a matter of debate - and other scientists have criticized Haggerty's theory; posing the question that if the black diamonds fell as a meteorite, why are most of them so small in size, and why are there no large pieces? Haggerty has stated that "No satisfactory explanation exists for their origins. They may reveal a new geological source for diamond formation that hasn't been recognized." 
The Cause of Color of Black Diamonds
The black color of black diamonds is believed by mineralogists to be caused by inclusions of dark colored material. Some examples were found to contain graphite, however in the scientific study (mentioned above) of black diamonds originating in Siberia, the stones with the most intense black color were found to be those with predominantly magnetite inclusions. Dark grey stones were found to contain inclusions of hematite and native iron.  It has been stated elsewhere that the cause of color of the black diamond is that its unique crystal structure absorbs light. 
Are black diamonds harder or tougher than regular diamonds?
It has been widely agreed upon for at least a century - possibly much longer - that black diamonds are slightly harder or tougher than the regular diamonds, and it has been said since old times by lapidaries and mineralogists that they can only be polished by their own kind, as ordinary diamonds will not cut them. For example, the "Report of the State Mineralogist" appearing in the 1883 California Journal of Mines and Geology states "Black diamonds are sometimes called "carbonate," or "carbonado." They are even harder than the crystallized stones." 
For this reason, and because of their lack of sparkle, their chief use in past times was in drilling and cutting; while clear "gem" diamonds were considered more precious and saved for jewelry. Although claims of slightly superior mechanical characteristics of natural black diamonds (unless severely flawed) over natural diamonds are widespread and generally accepted , much of the modern scientific research on superhard materials has focused on synthetic materials; and synthetic polycrystalline diamonds have been created with greater hardness / toughness than natural diamonds of any variety.
The general theory as to black diamonds' superior mechanical characteristics is that they are made of diamond with ostensibly the same hardness as clear diamond, but as they are formed from a mass of tiny crystals, they do not have the "cleavage planes" which allow clear, monocrystalline diamonds to be split. In gemology, hardness and toughness are different measures - regular clear diamond is extremely hard (can scratch other objects) but is not that tough (can be shattered due to cleavage planes).
Another theory suggests that black diamonds have the same hardness as other diamonds, but that due to their polycrystalline nature, they have more "cutting edges" and are therefore more effective in cutting other material. 
The Black Diamond in Couture
The black diamond has enjoyed a recent surge in popularity and renewed consideration in the world of haute couture. As a fashion statement, the black diamond epitomizes dark glamour and the seductive, neo-gothic allure. Black has always been the color of mysterious sophistication - and black diamonds are said to be especially popular with male wearers and anyone who prefers a tone of understatement or something slightly "edgy". Jewelers often use black diamonds as stones set in men's tungsten rings, earrings, cufflinks or other jewelry pieces to provide this "edgy" look.
Faceted black diamonds are said to have a unique luster owing to the fact that light is almost completely absorbed by them. Black diamonds are opaque, and so the cut parameters that are so important to creating the brilliance and fire of clear diamonds, are of no consequence. However, black diamonds still have the "adamantine" luster of transparent diamonds - and color is still a factor: Some black diamonds are "pure" black, while others are greyish or may have colorless or grey areas. 
The most noted cutter of black diamonds in the world is undoubtedly Fawaz Gruosi. He has been called the "King of black diamonds" and is reported to have created over 4,000 pieces of black diamond jewelry.  Gruosi started the de Grisogono brand in 1993 in Geneva, and is accredited with single-handedly creating the modern popularity for black diamond jewelry. Prior to his work, the black diamond, very rare and difficult to cut, had fallen from popularity and had been disregarded since its previous heyday in the 1930's. 
Despite its rarity, black diamond is generally less expensive than "regular" clear diamond. Perhaps this is because it lacks the sparkle and fire of the clear diamond, and is less infinitely gradable: It cannot be measured by the standard "four C's" of brilliant diamonds - Clarity, Color, Cut, Carat. However, since the 1990's and the work of Gruosi, the price of the black diamond has risen dramatically.
Black Diamond Treatments
Some black diamonds appearing on the market have been treated, owing to the rarity of natural black diamonds. Irradiation and high temperature annealing have been known to be used to color a diamond black. Several tests exist to distinguish natural black diamonds from treated stones: Bright light, such as that from a flashlight, can show a green or brown tint to treated diamonds, whereas natural black diamonds still appear black; Natural black diamonds show even color under magnification; and in some cases, a change in electrical conductivity can show that a stone has been treated. 
The World's Largest Black Diamonds
The biggest and most famous cut black diamond in the world is said to be the Spirit of de Grisogono diamond - pictured at the top of the page. Weighing in at a mighty 587 carats before cutting, it was cut into a 312.24 carat gemstone - the fifth largest cut diamond of any kind in the world. The Spirit of de Grisogono was discovered in the 20th century in a West African mine. It was cut by the Swiss jeweler De Grisogono, and set into a white gold ring together with 702 small white diamonds weighing a total of 36.69 carats.
Other famous black diamonds include: The Black Star of Africa, weighing 202 carats; the emerald cut Table of Islam, which weighs 160.18 carats; the Gruosi, a 115.34 heart-cut black diamond and the largest heart-cut diamond in the world; the Korloff Noir, an 88 carat stone cut from a 421 carat rough; the cushion-cut Black Orlov, also known as the Eye of Brahma, said to have been once owned by Russian Princess Nadia Vyegin-Orlov in the 18th century, weighing 67.50 carats and reported to have been re-cut from a stone once weighing 195 carats; and the 33.74 carat pear-cut Amsterdam.   
There are reports of gigantic black diamonds from days gone by: The Pharmaceutical Journal of September 28th, 1895 reported a 3,073 carat black diamond which had recently been shown to the Paris Academy of Sciences by a Mr. Moissan [presumably Henri Moissan (September 28, 1852, Paris – February 20, 1907) after whom Moissanite was named]. The black diamond was said to have a hardness greater than that of the brilliant and to be without flaws. The article also stated that this stone was larger than the three largest black diamonds discovered up to that time; 620, 810 and 1,700 carats - presumably these weights were for uncut stones. 
The article from the Pharmaceutical Journal is reproduced below; but what became of these four stones? Yet another mystery surrounds the black diamond!
At 3,073 carats - was this the largest Black Diamond ever discovered in the World?
A little more information on Moissan's giant black diamond is available from other publications of the day: the "Proceedings of the American Pharmaceutical Association, Volume 44" of 1896 states that the stone was found "between the Rio a Rancador and the Brook das Bicas, Brazil", also that the stone was porous and had lost 19 grams (presumably evaporating moisture) since being removed from the earth.  The 1895-1896 "Annual report of the United States Geological Survey to the Secretary of the Interior" stated that the Brazilian Government had made a strong effort to obtain the stone for the national museum at Rio de Janeiro.  Elsewhere, the stone was described as having a rough surface, and under magnification appeared as though gas had escaped from it while it was still pasty. It does seem from all these articles that this stone actually existed - but where is it now? And where are the other stones mentioned in the Pharmaceutical Journal of 1895?
For our answer we must turn to Popular Mechanics of December 1931, which has a full article on the black diamond. The article has a photo of the "Largest Black Diamond Ever Found" - almost certainly the same stone - stating its weight to be 3,078 carats. The ultimate fate of this stone is discovered in the simple economic factors of the black diamond's toughness combined with its unattractiveness when rough: The large black diamonds of old were often used for drilling and not considered precious. The Popular Mechanics article states "You can't give them to your sweetheart, because she would believe you were presenting her with a nugget of charcoal.... Large stones are rare. A 3,078-carat carbon was broken up for the diamond drills used to develop the Mesabi range. The King of carbon has, in the past five years, purchased and broken stones ranging from 100 to 458 carats."  However, here is another mystery: Other black diamonds appear to have been used as jewels since prior to the 20th century; therefore if the 3,078 carat stone was flawless, as Mr. Moissan, a noted mineralogist, states - why would it have been broken up? Fashion trends aside, it seems difficult to imagine that it was worth more to industrial drilling than it would have been to the gem industry - but perhaps that is exactly what happened.
History and Mythology of Black Diamonds
Illustration from Punch, 1851 - dry wit on the
subject of the preciousness of black diamond...
A ubiquitous modern falsehood surrounding the black diamond, is that it is said to be a "stone of reconciliation". Innumerable web pages writing articles about the black diamond in the last decade, state that in Mediaeval Italy it was believed that a black diamond, waved in the face of an angry spouse, would magically bring calm and restoration of peace to a troubled scenario. This is indeed curious when we consider that most scientific sources state the black diamond to have been first discovered in the 19th century! 
Searching for the phrase "stone of reconciliation" in older texts, we find that everything prior to the 19th century states that it was not the black diamond that is the stone of reconciliation, but the "regular", sparkling brilliant diamond! Turning to G.F. Kunz, noted expert on the history and lore of gems, we find that this quality of diamond traces back to Rueus, who stated it in his 1566 work De Gemmis.  It appears that the tale is older still: Several sources mention that Dioscorides, the Greek Physician of 40-90AD, called diamond "a Precious stone of reconciliation and of love." This undoubtedly comes from his famous De Materia Medica, a five volume medicinal treatise.
Not a single one of the old texts or sources I found mentions black diamond in this regard: All refer to the brilliant diamond - leading one to reflect that not a single one of the modern internet articles checked their sources; and on the statement of Winston Churchill, never more true than in the internet age, that "A lie gets halfway round the world before the truth has a chance to get its pants on".
Black diamond was little mentioned in old literature. It is scarce even now and seems to have been extremely scarce, if known at all, prior to the 19th century. Most modern scientific sources state that it was first found around 1845 in Brazil  , however this is highly controversial in the light of some earlier writings - as will be seen below.
The first mention of the phrase "black diamond" I have found is from the 1609 "Harangues et actions publiques des plus rares esprits de nostre temps" ("Speeches and public actions of the rarest spirits of our time.") This instance does not describe the stone itself, but the phrase is used metaphorically in describing the eyes of a person. However, one is tempted to suggest that the black diamond must have been known at least by fable to the author; how would one have been able to draw a visual parallel to a stone that was not known to exist?
The earliest actual description of black diamond I have found so far is from Joannon de Saint-Laurent's 1746 "Description Abregée du Fameux Cabinet de M. le Chevalier de Baillou, Pour Servir a L'Histoire Naturelle des Pierres Precieuses, Metaux, Mineraux et Autres Fossiles." ("Brief Description of the Famous Cabinet of the Chevalier Baillou, with Regard to the Natural History of Precious Stones, Metals, Minerals and Other Fossils.") This work lists numerous precious stones, and then states "Finally here we have the black Diamond, the hardest of all."  No further descriptive detail appears - and although it appears from the preceding text that the cabinet contained many faceted stones, it is unclear whether the stone in question was faceted.
The next description of black diamond I can find is from the 1763 "Commercium Philosophico-Technicum" of William Lewis, who wrote: "I have been favoured with a sight of this stone, and am assured that it is a true diamond. At a distance, it looks uniformly black; but on closer examination, it appears in some parts transparent, and in others charged with foulness, on which the black hue depends."  He also states on p.321 "I have been informed by a skilful jeweller that he had seen a black diamond, cut and set in a ring; though perhaps the examination made of it was not so rigorous as could be wished for determining its being truly of the diamond kind."
Of course, we must bear in mind that we do not have the benefit of scientific examination of these stones in order to verify that they were indeed black diamonds; however, further literary mentions all but assure us absolutely that the black diamond had been discovered by this time: Jean Claude de La Metherie, a mineralogist and geologist of note, writes in his 1795 "Théorie de la terre" ("Theory of the Earth"), "Le diamant noir est plus dur que le diamant blanc." - "The black diamond is harder than the white diamond".
The Encyclopaedia Britannica of 1797 mentions a Mr. Dutens, who related that he had seen a black diamond in Vienna in the collection of the prince de Lichtenstein.  This was Louis Dutens (1730-1812), a French writer whose Des Pierres Précieuses et des Pierres Fines (Precious Stones and Fine Stones) appeared in 1776; however turning to his work we are enlightened no further, as he simply states "J'en ai vu un noir dans la collection du Prince de Lichtenstein à Vienne" - "I saw a black one [diamond] in the collection of the Prince of Liechtenstein in Vienna."  G.F. Kunz, writing in 1917 in "Rings for the finger: from the earliest known times, to the present" describes in the collection of the famous Imperial Kunstgewerbe Museum (Museum of Arts and Crafts), Vienna, a sun-dial ring made in the 17th century, with a lid "studded with black diamond lozenges". Perhaps this is a coincidence, perhaps not: The faceting of one larger stone often creates several usable smaller pieces and, given the rarity of black diamond, it is not an impossible stretch to venture that these jewels may all have originated in the same stone. If it were a lesser writer describing the sun-dial ring, it would be easy to dismiss the lozenges as probably being made from another mineral altogether; however Kunz was a celebrated gemologist (who even had a gemstone - kunzite - named after him), rendering this possibility less likely.
It's another oft-repeated "internet myth" that the Duke of Wellington was an aficionado of black diamonds, and once owned one weighing 12.25 carats.  Which Duke of Wellington is not stated - but although most unqualified references to "The Duke of Wellington" refer to the first and most famous, Arthur Wellesley, it seems somewhat unlikely to have been him as he lived from 1769 to 1852.
There is, however, more: "The Lady's realm, Volume 12" of September 1902, describes the "most famous collection of jewels of modern times", that of the Duke of Richmond, to contain a black diamond that "...did duty for centuries as the eye of an Indian idol."  This diamond sounds suspiciously like the Black Orlov previously mentioned, a stone once nicknamed the 'Eye of Brahma' and said to have been prised from the statue of an Indian Deity. The Orlov, like many other diamonds, has controversial origins lost in the mists of time.
From all this it seems abundantly clear that a handful of black diamonds must have been known before the first recognized "official" discovery in 1845. It's possible that a handful were found many centuries before, and found their way from the treasuries of the Orient and Africa, through to the jewel collections of 18th century Europe. But the first chapter in the history of the black diamond is evidently entirely missing, save for a few scant inscriptions.
All things considered, it seems that after all this research, we can only say one thing about the black diamond for certain: That it exists. In every other regard, it seems, curiously, to resist study as fiercely as it resists the blade. The more we examine it, the less we seem to know - as every statement made about it seems to contradict another. Even after all this time, the black diamond still remains as perhaps the most mysterious gemstone on earth and one of the true enigmas of our time.
AN - March 2011
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Black Diamond - Sources Referenced:
 "An Investigation into the Cause of Color in Natural Black Diamonds from Siberia" - Gems & Gemology, Fall 2003
 "The Curious Lore of Precious Stones" - G.F. Kunz, 1913.
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