<P> Diamonds are extremely rare, with concentrations of at most parts per billion in source rock . Before the 20th century, most diamonds were found in alluvial deposits . Loose diamonds are also found along existing and ancient shorelines, where they tend to accumulate because of their size and density . Rarely, they have been found in glacial till (notably in Wisconsin and Indiana), but these deposits are not of commercial quality . These types of deposit were derived from localized igneous intrusions through weathering and transport by wind or water . </P> <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>

We know that diamonds did not form from coal because
find me the text answering this question