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
A crystallite is a small or even microscopic crystal which forms, for example, during the cooling of many materials. Crystallites are also referred to as grains.
Bacillite is a type of crystallite. It is rodlike with parallel longulites.
Stru ...
diamond consisting of diamond,
graphite, and
amorphous carbon Amorphous carbon is free, reactive carbon that has no crystalline structure. Amorphous carbon materials may be stabilized by terminating dangling-π bonds with hydrogen. As with other amorphous solids, some short-range order can be observed. Amorph ...
, with minor crystalline precipitates filling pores and occasional reduced metal inclusions. It is found primarily in
alluvial
Alluvium (from Latin ''alluvius'', from ''alluere'' 'to wash against') is loose clay, silt, sand, or gravel that has been deposited by running water in a stream bed, on a floodplain, in an alluvial fan or beach, or in similar settings. Alluv ...
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
Porosity or void fraction is a measure of the void (i.e. "empty") spaces in a material, and is a fraction of the volume of voids over the total volume, between 0 and 1, or as a percentage between 0% and 100%. Strictly speaking, some tests measure ...
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 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, 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
The Latin adverb ''sic'' (; "thus", "just as"; in full: , "thus was it written") inserted after a quoted word or passage indicates that the quoted matter has been transcribed or translated exactly as found in the source text, complete with any e ...
, and
Fe‑
Ni. No distinctive high-pressure minerals, including the hexagonal
carbon polymorph,
lonsdaleite
Lonsdaleite (named in honour of Kathleen Lonsdale), also called hexagonal diamond in reference to the crystal structure, is an allotrope of carbon with a hexagonal lattice, as opposed to the cubical lattice of conventional diamond. It is found ...
, have been found as inclusions in carbonados although such inclusions might be expected if carbonados formed by
meteorite 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. photo ...
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 ...
) 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
Bort, boart, or boort is an umbrella term used in the diamond industry to refer to shards of non- gem-grade/quality diamonds. In the manufacturing and heavy industries, "bort" is used to describe dark, imperfectly formed or crystallized diamonds ...
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 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.]
None of these hypotheses for carbonado formation had come into wide acceptance in the scientific literature by 2008. 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. 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 split
Split(s) or The Split may refer to:
Places
* Split, Croatia, the largest coastal city in Croatia
* Split Island, Canada, an island in the Hudson Bay
* Split Island, Falkland Islands
* Split Island, Fiji, better known as Hạfliua
Arts, enterta ...
into Brazil and the Central African Republic, assumed to be the only two known locations of carbonado-diamond deposits.
Largest cut diamond
The largest cut diamond in the world is a carbonado named ' The Enigma', weighing .
See also
*
*
*
*
*
*
*
*
*List of diamonds
Diamonds become famous typically for some combination of their size, color and quality. Diamonds occur naturally in many different colors, so the largest diamond of a particular color may not be large in absolute terms, but it may still be consid ...
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.
Carbonado Diamonds
at Gemone Diamond
Diamond
Diamond colors