The Popigai impact

An asteroid perhaps five to eight kilometres across strikes northern Siberia about 35.7 million years ago, excavating a crater around 100 kilometres wide — one of the two largest of the Cenozoic. The target rock is graphite-bearing gneiss, and the shock pressure converts the graphite directly into impact diamonds. The resulting deposit is estimated in the trillions of carats, exceeding all conventional reserves combined, though the crystals are small and industrial. The Soviet Union classified the site; it was made public in the 1990s.