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'{{short description|Impure form of polycrystalline diamond consisting of diamond, graphite, and amorphous carbon}} {{other uses}} {{Redirect|Black diamonds||Black Diamond (disambiguation){{!}}Black Diamond}} {{Infobox mineral |name = Carbonado |category = Native minerals |boxwidth = |boxbgcolor = #7da7d9 |image = Carbonado diamondites Bangui region, Central African Republic.jpg |alt = Sometimes cut as gemstones - often requiring lasers -, but have a granular appearance. Usually cracked in high-pressure presses for industrial usage. |caption = Three carbonados from the Central African Republic |formula = [[Carbon|C]] |molweight = {{val|12.01|ul=u}} |color = Typically black, can be grey, various shades of green and brown sometimes mottled. |habit = [[Polycrystalline]] |system = Isometric-hexoctahedral ([[cubic crystal system|cubic]]) |twinning = |cleavage = |fracture = Irregular torn surfaces |mohs = 10 |luster = [[Adamantine lustre|Adamantine]] |polish = Adamantine |refractive = |opticalprop = None |birefringence = None |dispersion = |pleochroism = None |streak = White |gravity = {{val|3.52|0.01}} |density = 3.5–{{val|3.53|ul=g/cm3}} }} '''Carbonado''', commonly known as '''black diamond''', is one of the [[toughness|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.<ref>{{cite book|last=Kroschwitz]|first=[executive editor, Jacqueline I.|title=Kirk-Othmer encyclopedia of chemical technology|year=2004|publisher=J. Wiley|location=Hoboken, N. J.|isbn=9780471484943|page=10|edition=5th}}</ref> It is found primarily in [[alluvium|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 [[porosity|porous]] than other diamonds. ==Unusual properties== Carbonado diamonds are typically [[pea]]-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 analysis|isotope analyses]] have been interpreted as documenting crystallization of carbonados about 3 billion years ago; yet carbonado is found in younger [[sedimentary rock]]s.<ref name=Heaney>{{cite journal|last1=Heaney|first1=P. J.|last2=Vicenzi|first2=E. P.|last3=De|first3=S.|title=Strange Diamonds: the Mysterious Origins of Carbonado and Framesite|journal=Elements|volume=1|page=85|year=2005|doi=10.2113/gselements.1.2.85|issue=2}}</ref> Mineral grains included within diamonds have been studied extensively for clues to diamond origin. Some typical diamonds contain inclusions of common [[mantle (geology)|mantle]] minerals such as [[pyrope]] and [[forsterite]], 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 (geology)|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 [[silicon|Si]], [[silicon carbide|SiC]], and [[iron|Fe]]‑[[nickel|Ni]]. No distinctive high-pressure minerals, including the hexagonal [[carbon]] [[polymorphism (materials science)|polymorph]], [[lonsdaleite]], have been found as inclusions in carbonados although such inclusions might be expected if carbonados formed by [[meteorite]] impact.<ref name=Heaney/> [[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).<ref name=Heaney/> Carbonado exhibits strong luminescence ([[photoluminescence]] and [[cathodoluminescence]]) induced by [[nitrogen]] and by [[vacancy (chemistry)|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,<ref>{{cite journal|author=Kagi, H., Sato, S., Akagi, T., and Kanda, H., 2007|title=Generation history of carbonado inferred from photoluminescence spectra, cathodoluminescence imaging, and carbon-isotopic composition|journal=American Mineralogist |volume=92|pages=217–224|year=2007|url=http://www.minsocam.org/msa/ammin/toc/Abstracts/2007_Abstracts/Jan07_Abstracts/Kagi_p217_07.pdf|doi=10.2138/am.2007.1957|bibcode=2007AmMin..92..217K}}</ref> an observation perhaps pertinent to the radiation hypothesis listed below. ==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]] 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.<ref name=r1/> # An origin in interstellar space, due to the impact of an asteroid, rather than being thrown from within an exploding star.<ref name=r1>{{cite journal |url=http://www.fiu.edu/~jgara002/research%20statement/carbonado/carbonado-infrared.htm |last1=Garai |first1=Jozsef |journal=The Astrophysical Journal |volume=653 |last2=Haggerty |pages=L153 |first2=Stephen E. |last3=Rekhi |first3=Sandeep |last4=Chance |first4=Mark |year=2006 |title=Infrared Absorption Investigations Confirm the Extraterrestrial Origin of Carbonado Diamonds |doi=10.1086/510451 |bibcode=2006ApJ...653L.153G |issue=2 |arxiv=physics/0608014 |url-status=dead |archiveurl=https://web.archive.org/web/20070809204752/http://www.fiu.edu/~jgara002/research%20statement/carbonado/carbonado-infrared.htm |archivedate=2007-08-09 }}. 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.</ref> None of these hypotheses for carbonado formation had come into wide acceptance in the scientific literature by 2008.<ref>{{cite journal|last1=Rondeau|first1=B|last2=Sautter|first2=V|last3=Barjon|first3=J|title=New columnar texture of carbonado: Cathodoluminescence study|journal=Diamond and Related Materials|volume=17|page=1897|year=2008|doi=10.1016/j.diamond.2008.04.006|issue=11|bibcode= 2008DRM....17.1897R }}</ref> 13C/12C ratios in carbonado are identical to those of local sediments. ===Extraterrestrial origin hypothesis=== Supporters of an extraterrestrial origin of carbonados propose that their material source was a [[supernova]] which occurred at least 3.8 billion years ago.<ref>{{Cite news|url=https://www.nytimes.com/1996/09/17/science/giant-black-diamonds-of-mysterious-origin-may-hail-from-space.html|title=Giant Black Diamonds Of Mysterious Origin May Hail From Space|last=Broad|first=William J.|date=1996-09-17|newspaper=The New York Times|issn=0362-4331|access-date=2016-09-20}}</ref><ref>{{Cite journal|last=Garai|first=Jozsef|last2=Haggerty|first2=Stephen E.|last3=Rekhi|first3=Sandeep|last4=Chance|first4=Mark|date=2006-12-20|title=Infrared Absorption Investigations Confirm the Extraterrestrial Origin of Carbonado-Diamonds|journal=The Astrophysical Journal|volume=653|issue=2|pages=L153–L156|doi=10.1086/510451|issn=0004-637X|arxiv=physics/0608014|bibcode=2006ApJ...653L.153G}}</ref> After coalescing and drifting through [[outer space]] for about one and a half billion years, a large mass fell to earth as a [[meteorite]] 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 [[plate tectonics|split]] into [[Brazil]] and the [[Central African Republic]], assumed to be the only two known locations of carbonado-diamond deposits (which is not an accurate representation of the distribution of carbonado diamondite, the poly-chrystaline form). == Largest cut diamond == The largest cut diamond in the world is a carbonado diamond named "The Enigma", weighing 555.55 carats.<ref>{{Cite web|date=2022-01-30|title=The Black Diamond, an Engima Weighing 555.55 Carats {{!}} GeoRarities|url=https://georarities.com/2022/01/30/the-black-diamond-an-engima-weighing-555-55-carats/|access-date=2022-01-31|language=en-US}}</ref> ==See also== {{Portal|Minerals}} *{{annotated link|Amsterdam Diamond}} *{{annotated link|Bort}} *{{annotated link|Korloff Noir}} *{{annotated link|Material properties of diamond}} *{{annotated link|Popigai crater#Diamond deposits|Popigai diamonds}} *{{annotated link|Sergio (carbonado)}} *{{annotated link|Spirit of de Grisogono Diamond}} *{{annotated link|Superhard material}} ==References== {{Reflist}} ==External links== {{Commons category|Carbonado (mineral)}} {{EB1911 poster|Carbonado}} {{Wiktionary}} *[https://www.nsf.gov/news/mmg/mmg_disp.cfm?med_id=59860&from=mn Photo of porous carbonado] at National Science Foundation *[https://www.pbs.org/wgbh/nova/diamond/sky.html Photo of glossy carbonado and article on possible extraterrestrial origins] at PBS Nova *[https://web.archive.org/web/20111228141115/https://www.sciencedaily.com/videos/2007/0612-mystery_diamonds.htm Mystery Diamonds: Geoscientists Investigate Rare Carbon Formation] ScienceDaily (June 1, 2007) Story *[https://www.sciencedaily.com/releases/2007/01/070109172003.htm Diamonds From Outer Space: Geologists Discover Origin Of Earth's Mysterious Black Diamonds] ScienceDaily (January 9, 2007) Story. [[Category:Diamond]] [[Category:Diamond colors]]'