Carbonado (Java)
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Carbonado, commonly known as black diamond, is one of the toughest forms of natural diamond. It is an impure, high-density, micro-porous form of polycrystalline diamond consisting of diamond, graphite, and amorphous carbon, with minor crystalline precipitates filling pores and occasional reduced metal inclusions. Titanium nitride (TiN, osbornite) has been found in carbonado. It is found primarily in alluvial deposits where it is most prominent in mid-elevation equatorial regions such as Central African Republic and in Brazil, where the vast majority of carbonado diamondites have been found. Its natural colour is black or dark grey, and it is more porous than other diamonds.


Unusual properties

Carbonado diamonds are typically
pea The pea is most commonly the small spherical seed or the seed-pod of the flowering plant species ''Pisum sativum''. Each pod contains several peas, which can be green or yellow. Botanically, pea pods are fruit, since they contain seeds and d ...
-sized or larger porous aggregates of many tiny black crystals. The most characteristic carbonados are mined in the Central African Republic and in Brazil, in neither place associated with kimberlite, the source of typical gem diamonds. Lead isotope analyses have been interpreted as documenting crystallization of carbonados about 3 billion years ago; yet carbonado is found in younger sedimentary rocks. Mineral grains included within diamonds have been studied extensively for clues to diamond origin. Some typical diamonds contain inclusions of common
mantle A mantle is a piece of clothing, a type of cloak. Several other meanings are derived from that. Mantle may refer to: *Mantle (clothing), a cloak-like garment worn mainly by women as fashionable outerwear **Mantle (vesture), an Eastern Orthodox ve ...
minerals such as pyrope and
forsterite Forsterite (Mg2SiO4; commonly abbreviated as Fo; also known as white olivine) is the magnesium-rich end-member of the olivine solid solution series. It is isomorphous with the iron-rich end-member, fayalite. Forsterite crystallizes in the orthor ...
, but such mantle minerals have not been observed in carbonado. In contrast, some carbonados contain authigenic inclusions of minerals characteristic of the Earth's crust; the inclusions do not necessarily establish formation of the diamonds in the crust, because while the obvious crystal inclusions occur in the pores that are common in carbonados, they may have been introduced after carbonado formation. Inclusions of other minerals, rare or nearly absent in the Earth's crust, are found at least partly incorporated in diamond, not just in pores: among such other minerals are those with compositions of Si, SiC, and FeNi. No distinctive high-pressure minerals, including the hexagonal carbon
polymorph Polymorphism, polymorphic, polymorph, polymorphous, or polymorphy may refer to: Computing * Polymorphism (computer science), the ability in programming to present the same programming interface for differing underlying forms * Ad hoc polymorphi ...
, lonsdaleite, have been found as inclusions in carbonados although such inclusions might be expected if carbonados formed by
meteorite A meteorite is a solid piece of debris from an object, such as a comet, asteroid, or meteoroid, that originates in outer space and survives its passage through the atmosphere to reach the surface of a planet or Natural satellite, moon. When the ...
impact. Isotope studies have yielded further clues to carbonado origin. The carbon isotope value is very low (little carbon‑13 compared to carbon‑12, relative to typical diamonds). Carbonado exhibits strong luminescence (
photoluminescence Photoluminescence (abbreviated as PL) is light emission from any form of matter after the absorption of photons (electromagnetic radiation). It is one of many forms of luminescence (light emission) and is initiated by photoexcitation (i.e. photon ...
and
cathodoluminescence Cathodoluminescence is an optical and electromagnetic phenomenon in which electrons impacting on a luminescent material such as a phosphor, cause the emission of photons which may have wavelengths in the visible spectrum. A familiar example is th ...
) induced by nitrogen and by vacancies existing in the crystal lattice. Luminescence halos are present around radioactive inclusions, and it is suggested that the radiation damage occurred after formation of the carbonados, an observation perhaps pertinent to the radiation hypothesis listed below.


Toughness vs. hardness

Carbonado’s polycrystalline texture makes it more durable than a monocrystalline diamond. It is the same hardness as other types of diamond, but it is much tougher. Its polycrystalline texture allows a single abrasive granule to present multiple crystallographic orientations of the diamond crystal at the cutting surface and the hardest orientation does the most aggressive cutting. Cutting tools made with carbonado last longer and require less maintenance. Carbonado was recognized as an abrasive in the 1800s and was more highly valued for its cutting and grinding effectiveness over other varieties of diamond. The problem with carbonado is its rarity. It is only found in two countries, and total worldwide production has only been a few tons. Carbonado is not an important commodity in today's abrasive market. In the late 1800s, when De Beers was developing their diamond mines in South Africa, they preferred carbonado over their own diamonds for diamond drilling. Gardner F. Williams, General Manager of De Beers Consolidated Mines, Ltd. lamented: "Round or shot boart is found in the mines at Kimberley and is very valuable for use in diamond drilling since the Brazilian carbonado has become so scarce."


Hypotheses for origin

The origin of carbonado is controversial, and some proposed hypotheses are as follows: # Direct conversion of organic carbon under high-pressure conditions in the Earth's interior, the most common hypothesis for diamond formation #
Shock metamorphism Shock metamorphism or impact metamorphism describes the effects of shock-wave related deformation and heating during impact events. The formation of similar features during explosive volcanism is generally discounted due to the lack of metamorp ...
induced by meteoritic impact at the Earth's surface # Radiation-induced diamond formation by spontaneous fission of uranium and thorium # Accumulated local formation in reduced organic-rich sediment over long geologic periods due to pyrometamorphic-rapid processes associated with long-duration superbolt lightning strikes, known to have similar global distribution as carbonado diamondite deposits at similar elevations. # Formation inside an earlier-generation giant star in our area, that long ago exploded in a supernova. # An origin in interstellar space, due to the impact of an asteroid, rather than being thrown from within an exploding star.. This study suggested that infrared absorption spectra of carbonado are similar to diamonds of extraterrestrial origin; selected significant peaks are due to trace abundances of the elements nitrogen and hydrogen. The researchers concluded with the assumption that the mineral necessarily formed in an interstellar environment. In this sense, carbonado are theorized to be akin to carbon-rich cosmic dust, likely having formed in an environment near carbon stars. The diamonds were suggested to have been fragments of a body of asteroid size that subsequently fell to Earth as meteorites. The origin of carbonado is still under debate.


Extraterrestrial origin hypothesis

Supporters of an extraterrestrial origin of carbonados such as Stephen Haggerty propose that their material source was a
supernova A supernova is a powerful and luminous explosion of a star. It has the plural form supernovae or supernovas, and is abbreviated SN or SNe. This transient astronomical event occurs during the last evolutionary stages of a massive star or when ...
which occurred at least 3.8 billion years ago. After coalescing and drifting through outer space for about one and a half billion years, a large mass fell to earth as a
meteorite A meteorite is a solid piece of debris from an object, such as a comet, asteroid, or meteoroid, that originates in outer space and survives its passage through the atmosphere to reach the surface of a planet or Natural satellite, moon. When the ...
approximately 2.3 billion years ago. It possibly fragmented during entry into the Earth's atmosphere and impacted in a region which would much later split into Brazil and the Central African Republic, assumed to be the only two known locations of carbonado-diamond deposits. The presence of osbornite, which only forms under very reducing conditions and at very high temperatures, argues for an extraterrestrial origin.


Largest cut diamond

The largest cut black diamond in the world is a carbonado named ' The Enigma', weighing .


See also

* * * * * * * * * List of diamonds


References


External links

{{Wiktionary
Photo of porous carbonado
at National Science Foundation

at PBS Nova * ttps://web.archive.org/web/20111228141115/https://www.sciencedaily.com/videos/2007/0612-mystery_diamonds.htm Mystery Diamonds: Geoscientists Investigate Rare Carbon FormationScienceDaily (June 1, 2007) Story
Diamonds From Outer Space: Geologists Discover Origin Of Earth's Mysterious Black Diamonds
ScienceDaily (January 9, 2007) Story. Diamond Diamond colors