Diamond education/History Part five
Diamond and meteorite impact craters
Microdiamonds are very small diamonds, they have been found in meteorite impact craters. Such impact events create shock zones of high pressure and temperature suitable for diamond formation. Microdiamonds can be used as one indicator of ancient impact craters.
Extraterrestrial diamonds
Not exactly all diamonds found on earth originated here. About 3 billion years ago a type of carbonado diamonds that is found in South America and Africa was deposited their via an asteroid impact.(not formed from the impact)
Surfacing
Diamond-bearing rock is brought close to the surface through deep-origin volcanic eruptions. The magma for such a volcano must originate at a depth where diamonds can be formed. The pipes are open during surface eruption. Not all pipes contain diamonds, and even fewer contain enough diamonds to make mining economically viable.
Certain indicator minerals typically occur within diamondiferous kimberlites and are used as mineralogic tracers by prospectors, who follow the indicator trail back to the volcanic pipe, which may contain diamonds.
Once diamonds have been transported to the surface by magma in a volcanic pipe, they may erode out and be distributed over a larger area. A volcanic pipe containing diamonds is known as a primary source of diamonds. Secondary sources of diamonds include all areas where a significant number of diamonds, eroded out of their kimberlite or lamproite matrix, accumulate because of water or wind action. These include alluvial deposits and deposits along existing and ancient shorelines, where loose diamonds tend to accumulate because of their approximate size and density. Diamonds have rarely been found in deposits left behind by glaciers.