Radiation-induced diamond formation by spontaneous fission of uranium and thorium.Shock metamorphism induced by meteoritic impact at the Earth's surface.Direct conversion of organic carbon under high-pressure conditions in the Earth's interior, the most common hypothesis for diamond formation.The origin of carbonado is controversial, and some proposed hypotheses are as follows: 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 Williams, General Manager of De Beers Consolidated Mines, Ltd. 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. Carbonado is not an important commodity in today's abrasive market. It is only found in two countries, and total worldwide production has only been a few tons.
The problem with carbonado is its rarity. 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. 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.Ĭutting tools made with carbonado last longer and require less maintenance. It is the same hardness as other types of diamond, but it is much tougher. 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.Ĭarbonado’s polycrystalline texture makes it more durable than a monocrystalline diamond. Ĭarbonado exhibits strong luminescence ( photoluminescence and cathodoluminescence) induced by nitrogen and by vacancies existing in the crystal lattice. The carbon isotope value is very low (little carbon‑13 compared to carbon‑12, relative to typical diamonds). Isotope studies have yielded further clues to carbonado origin. No distinctive high-pressure minerals, including the hexagonal carbon polymorph, lonsdaleite, have been found as inclusions in carbonados although such inclusions might be expected if carbonados formed by meteorite impact. 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 Fe‑ Ni. 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. Some typical diamonds contain inclusions of common mantle minerals such as pyrope and forsterite, but such mantle minerals have not been observed in carbonado. Mineral grains included within diamonds have been studied extensively for clues to diamond origin. Lead isotope analyses have been interpreted as documenting crystallization of carbonados about 3 billion years ago yet carbonado is found in younger sedimentary rocks.
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. Its natural colour is black or dark grey, and it is more porous than other diamonds.Ĭarbonado diamonds are typically pea-sized or larger porous aggregates of many tiny black crystals. 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. 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. Typically black, can be grey, various shades of green and brown sometimes mottled.Ĭarbonado, commonly known as black diamond, is one of the toughest forms of natural diamond. Three carbonados from the Central African Republic
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