I remember reading a short story way back, about an explorer who comes back from Russia and claims the existence of a gigantic diamond (yes, a single one) somewhere in Siberia. The diamond was formed when an meteorite hit a coal deposit, and the resulting heat and pressure turned the carbon into a giant "diamond lake".

Well, this just in:

Russia has declassified a large deposit of super hard diamonds which are twice harder than usual ones. The sensational statement was made by Novosibirsk scientists of the Institute of Geology and Mineralogy at the Siberian Branch of the Russian Academy of Sciences.

The deposit is located on the border of the Krasnoyarsk region and Yakutia in the Popigai crypto-explosion structure- a hundred kilometres’ meteorite crater formed 35 million years ago. Back in the 1970s, Soviet geologists discovered there the first extra hard "diamonds" - impact diamonds having unusual features. They were two times harder than regular ones and had a different structure. (source)

Via BoingBoing