Carbonado is a natural polycrystalline diamond found in alluvial deposits in the Central African Republic and Brazil. This diamond looks black and is highly porous. 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. A polycrystalline material is made of several smaller crystallites. ... A scattering of round-brilliant cut diamonds shows off the many reflecting facets. ... Bold textLink title Headline text Alluvium is soil land deposited by a river or other running water. ... A Mantle is a piece of clothing, similar to a robe but open on the front side and often sleeveless. ... General Name, Symbol, Number carbon, C, 6 Chemical series nonmetals Group, Period, Block 14, 2, p Appearance black (graphite) colorless (diamond) Atomic mass 12. ... Isotopes are forms of an element whose nuclei have the same atomic number–-the number of protons in the nucleus--but different atomic masses because they contain different numbers of neutrons. ... Luminescence is light not generated by high temperatures alone. ... General Name, Symbol, Number nitrogen, N, 7 Chemical series nonmetals Group, Period, Block 15, 2, p Appearance colorless Atomic mass 14. ... In mineralogy and crystallography, a crystal structure is a unique arrangement of atoms in a crystal. ... Radioactive decay is the set of various processes by which unstable atomic nuclei (nuclides) emit subatomic particles. ...

The origin of carbonado is controversial, but several hypotheses have been proposed:

1. Direct conversion of organic carbon under high-pressure conditions (the Earth's interior)
2. Shock metamorphism induced by meteoritic impact at the Earth's surface
3. Radiation-induced diamond formation by spontaneous fission of uranium and thorium

These hypotheses, however, still have several problems. First, if carbonado were formed by phase transformation of organic graphite inside the Earth, carbonado would be found all over the world. But carbonado appears only in the Central African Republic and Brazil. Second, the formation process of a shock metamorphic event also has problems, because shock-induced natural polycrystalline diamonds usually have hexagonal diamond (lonsdaleite) inside the samples and the presence of hexagonal diamond inside carbonado has not been reported. The third hypothesis, radiation-induced diamond formation, is questionable as well, because the energy of radiogenic fission is too small to create polycrystalline diamond of the large grain size of carbonado (up to 500 micrometers). These diamonds are currently being used in various designs and settings in the jewelry industry. General Name, Symbol, Number carbon, C, 6 Chemical series nonmetals Group, Period, Block 14, 2, p Appearance black (graphite) colorless (diamond) Atomic mass 12. ... Metamorphism can be defined as the mineralogical, chemical and crystallographic changes in a solid-state rock, i. ... A burst of meteors A meteor is the visible path of a meteoroid that enters the Earths (or another bodys) atmosphere, commonly called a shooting star or falling star. ... Earth, also known as Terra, and (mostly in the 19th century) Tellus, is the third-closest planet to the Sun. ... A scattering of round-brilliant cut diamonds shows off the many reflecting facets. ... General Name, Symbol, Number uranium, U, 92 Chemical series actinides Group, Period, Block ?, 7, f Appearance silvery gray metallic Atomic mass 238. ... General Name, Symbol, Number thorium, Th, 90 Chemical series transition metals Group, Period, Block ?, 7, f Appearance silvery white Atomic mass 232. ... Graphite (named by Abraham Gottlob Werner in 1789, from the Greek γραφειν: to draw/write, for its use in pencils) is one of the allotropes of carbon. ... Lonsdaleite is a hexagonal polymorph of the carbon allotrope diamond, believed to form when meteoric graphite falls to Earth. ... A micrometre (American spelling: micrometer), symbol µm, is an SI unit of length. ...