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Carbonado, commonly known as the 'Black Diamond,' is a natural polycrystalline diamond found in alluvial deposits in the Central African Republic and Brazil. Its natural colour is black or dark grey, and it is more porous than other diamonds.

Unlike other natural polycrystalline diamonds, carbonado has no mantle-derived inclusions and its carbon isotope value is very low. Additionally, carbonado exhibits strong luminescence (photoluminescence and cathodoluminescence) induced by nitrogen and by vacancies existing in the crystal lattice. Analysis of the luminescence suggests that radioactive inclusions existed in the formation process of carbonado. These and other characteristics that separate the carbonado from other diamonds have led to questions as to the carbonado's origin.

The origin of carbonado is controversial, with several hypotheses proposed:

1. Direct conversion of organic carbon under high-pressure conditions (the Earth's interior.) This is the standard, geological process of diamond formation. The problem with this hypothesis is that, were carbonado formed by phase transformation of organic graphite inside the Earth, they would be found all over the world. However, carbonado appears only in the Central African Republic and Brazil, in areas that are far from other diamond deposits.
2. Shock metamorphism induced by meteoritic impact at the Earth's surface. According to this hypothesis, carbonado were created by meteoric impact. The problem with this hypothesis is that shock-induced natural polycrystalline diamonds usually have hexagonal diamond (lonsdaleite) inside the samples, and carbonado does not.
3. Radiation-induced diamond formation by spontaneous fission of uranium and thorium. The problem with this hypothesis is that the energy of radiogenic fission is too small to create polycrystalline diamonds of the large grain size of carbonado (up to 500 micrometers).

A team of U.S. geologists have published evidence relating to a different origin of these black diamonds: interstellar space. They have found that black diamonds contain trace elements of nitrogen and hydrogen which they claim are sure indicators of an extraterrestrial origin.

The study published in 2006 by Stephen Haggerty and Jozsef Garai, of Florida International University, analyzed the hydrogen in black diamond samples using infrared-detection instruments at the Brookhaven National Laboratory. The researchers found that the chemical properties of carbonado indicated that the mineral formed in a supernova explosion that took place prior to the formation of our Solar System.

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 eventually incorporated into solid bodies that subsequently fell to Earth as meteorites.^[1]

• Photo of porous carbonado at National Science Foundation
• Photo of glossy carbonado and article on possible extraterrestrial origins at PBS Nova

• Material properties of diamond
• Amsterdam Diamond

1. ^ Garai et al. 2006 Astrophysical Journal Letters, vol 653, pp L153-L156 http://www.fiu.edu/~jgara002/research%20statement/carbonado/carbonado-infrared.htm Infrared Absorption Investigations Confirm the Extraterrestrial Origin of Carbonado-Diamonds