Meteor strikes are one of the most surprising causes of diamond formation on the surface of the Earth. Most sources of diamonds today were formed by the geological processes of the Earth itself, and meteorites sometimes carry small quantities of tiny diamonds to Earth from elsewhere in the solar system. However, in a few cases, large meteor strikes have actually formed diamond deposits. The Popigai crater in Siberia, for instance, is now known to contain a massive diamond deposit, the true scale of which was concealed for many years by the Soviet and Russian governments.
Geologists and chemists understand how diamonds form quite well. According to material on diamonds provided by the University of California, Berkeley, diamonds form from carbon deposits when they are subjected to almost phenomenal amounts of heat and pressure - around 1,800 degrees Fahrenheit (1,000 degrees Celsius) and 50 kilobars of pressure (about 50,000 times atmospheric pressure at sea level). In most cases, these conditions are found only deep beneath the surface of the Earth. In these cases, the diamonds found on the surface were transported there by volcanic eruptions.
Diamond formation is not limited to the subterranean parts of Earth, however: it's simply that the right conditions of heat and pressure are most often found there. More rarely, the same conditions can briefly be created on the surface of the Earth by a meteor strike, which also involves a massive release of energy and, briefly, an enormous level of pressure. According to American Museum of Natural History minerals curator George Harlow, who was interviewed by Popular Mechanics, a large meteor strike is "like setting off a nuclear bomb, but bigger." Such strikes are rare, but their consequences are well known. One impact by an asteroid several miles wide in Central America 65 million years ago, at Chicxulub, carved out a crater over 100 miles in diameter. Even meteorites which break up in the atmosphere, as occurred over Tunguska, Russia, in 1908 and over the Ural Mountains, also in Russia, in 2013 can easily release as much energy as a nuclear bomb.
According to the UK Natural History Museum, some asteroids which enter the Earth's atmosphere have already undergone such traumatic collisions before, and carry diamonds formed during those collisions with them to Earth. Cases where large diamond deposits have formed as a result of the meteor strike itself are rarer - it requires a large impact into an area containing a substantial amount of carbon, in the form of graphite (the same material used for pencil leads). The most celebrated example today is the Popigai crater, once again in Siberia, but diamonds have also been found in the Ries Crater in Germany and Vredefort in South Africa. At Popigai, this is believed to have happened about 35 million years ago, during the Eocene, and the impact was large enough to carve out a 60-mile-wide crater about 500 miles from present-day Norilsk. The resulting shock wave would have triggered diamond formation.
The rich diamond deposits under Popigai were discovered and worked by the Stalinist regime using prison labour, and the true extent of the reserves was apparently concealed by the Russian government until the early 21st century. In 2012, there was widespread press coverage of an announcement by the Russian government that Popigai might contain "trillions of carats" of diamonds - that is, several hundred thousand tons. However, much of the Popigai deposit would be useful only for industrial purposes, thus limiting its market value.