<P> Most diamonds come from the Earth's mantle, and most of this section discusses those diamonds . However, there are other sources . Some blocks of the crust, or terranes, have been buried deep enough as the crust thickened so they experienced ultra-high - pressure metamorphism . These have evenly distributed microdiamonds that show no sign of transport by magma . In addition, when meteorites strike the ground, the shock wave can produce high enough temperatures and pressures for microdiamonds and nanodiamonds to form . Impact - type microdiamonds can be used as an indicator of ancient impact craters . Popigai crater in Russia may have the world's largest diamond deposit, estimated at trillions of carats, and formed by an asteroid impact . </P> <P> A common misconception is that diamonds are formed from highly compressed coal . Coal is formed from buried prehistoric plants, and most diamonds that have been dated are far older than the first land plants . It is possible that diamonds can form from coal in subduction zones, but diamonds formed in this way are rare, and the carbon source is more likely carbonate rocks and organic carbon in sediments, rather than coal . </P> <P> Diamonds are far from evenly distributed over the Earth . A rule of thumb known as Clifford's rule states that they are almost always found in kimberlites on the oldest part of cratons, the stable cores of continents with typical ages of 2.5 billion years or more . However, there are exceptions . The Argyle diamond mine in Australia, the largest producer of diamonds by weight in the world, is located in a mobile belt, also known as an orogenic belt, a weaker zone surrounding the central craton that has undergone compressional tectonics . Instead of kimberlite, the host rock is lamproite . Lamproites with diamonds that are not economically viable are also found in the United States, India and Australia . In addition, diamonds in the Wawa belt of the Superior province in Canada and microdiamonds in the island arc of Japan are found in a type of rock called lamprophyre . </P> <P> Kimberlites can be found in narrow (1--4 meters) dikes and sills, and in pipes with diameters that range from about 75 meters to 1.5 kilometers . Fresh rock is dark bluish green to greenish gray, but after exposure rapidly turns brown and crumbles . It is hybrid rock with a chaotic mixture of small minerals and rock fragments (clasts) up to the size of watermelons . They are a mixture of xenocrysts and xenoliths (minerals and rocks carried up from the lower crust and mantle), pieces of surface rock, altered minerals such as serpentine, and new minerals that crystallized during the eruption . The texture varies with depth . The composition forms a continuum with carbonatites, but the latter have too much oxygen for carbon to exist in a pure form . Instead, it is locked up in the mineral calcite (Ca C O). </P>

Where do most of the worlds diamonds come from