The origin of the black diamond

Carbonado, meaning black diamond, is a kind of naturally formed polycrystalline diamond originating from alluvial deposits in the Central African Republic and Brazil and is also found in the Kozelski volcano in Kamchatka. Their natural colour is black or dark grey and they are more porous than other diamonds.

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Unusual properties
Unlike other natural polycrystalline diamonds, carbonado has no inclusions from the earth’s mantle and a low δ-C13 value (the ratio of carbon isotopes). Carbonado also exhibits a very strong luminescence (photoluminescence and cathodoluminescence) caused by nitrogen and by vacancies in the crystal lattice. Analysis of luminescence suggests that there was radioactive uptake in the formation process of carbonado. These and other characteristics that distinguish carbonado from other diamonds have raised questions about the origin of carbonado.

Direct conversion of organic carbon in the high pressure conditions inside the Earth. This is the standard process by which diamonds are formed in nature. The problem with this hypothesis is that if carbonado is created by a phase transition in organic graphite, it should be found all over the world. However, carbonado only occurs in the Central African Republic and Brazil, areas where no other types of diamonds are found.
Shock metamorphosis during meteorite impacts on the earth’s surface. What is not true is that in shock metamorphosis usually hexagonal diamonds (lonsdaleite) are formed, not carbonado.

Radiogenic diamond formation by natural radioactive decay of uranium and thorium. The problem with this hypothesis is that the energy of these decay reactions is too small to form carbonado in the size in which it is found (up to 500 μm).
Carbonado contains traces of nitrogen and hydrogen, analysis of the hydrogen isotopes has shown that they originate from supernovas. This could mean that carbonado may have an extraterrestrial origin.[1] The idea is that carbonado is related to carbon-rich interstellar matter and is formed near carbon stars. After that, it was incorporated into a meteorite that hit Earth later.

Fluorescence is a special case of luminescence. It is a physical phenomenon in which an atom absorbs a high-energy photon, causing an electron to go into an excited state and then fall back to the ground state under emission of a photon of lower energy (Longer wavelength).

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The concept of fluorescence comes from fluorite: a mineral that consists of the salt calcium fluoride (CaF2), a known fluorescent substance.

Types of fluorescence
Fluorescence in the region of ultraviolet radiation and visible light is caused by excited states of an electron in an atom: the energy differences between the paths of the binding electrons there are in the order of a few electron volts (eV). A small change in the atom will change the energy levels a little, and therefore also the wavelength at which the fluorescence occurs. These effects can be used to identify the molecules.

Fluorescence (X-ray fluorescence) can also occur for X-rays, but these are the inner, most tightly bound electrons of heavier atoms. The energy differences there are of the order of a few to tens of keV. These energy levels do not change measurably when the atom is incorporated into a molecule, so these fluorescence phenomena can be used to determine the types of atoms that make up a sample independently of the chemical bonds. This technique is called X-ray fluorescence spectrometry.

Application
There are both naturally occurring and synthesized materials with fluorescent properties. The best-known examples are the fluorescent tube and the low-energy light bulb. Here ultraviolet radiation is generated by an electrical discharge in a diluted gas in a tube. This UV light will then be converted into visible light by a powder applied to the inside of the tube. Such powders are often called phosphors although they do not contain phosphorus.

The same principle is used when checking the authenticity of banknotes. In this case, the banknotes are equipped with fluorescent substances, which can be made visible by means of a UV lamp.

Furthermore, fluorescent colors are used in clothing that should stand out in traffic (the orange vests of road workers), and in yellow, green and orange pens to mark text. Because such colours convert short wavelengths of blue light into colours with longer wavelengths, under some lighting conditions (disco, UV light) they do indeed appear to emit light themselves. 

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