Slate: Some evidence of this has been found, but until now the science has been shaky. by Phil Plait
'A team of scientists from around the globe has just published new results, though, showing that diamonds can indeed be formed in just this way, and let me be honest: There is nothing about this story that isn’t totally cool. Lasers! X-rays! Asteroid impacts! Precious gems!
'So yes, first, let’s talk diamonds for a minute. You may already know diamonds are a form of carbon. When you think of carbon you probably picture a soft, black material like coal. That’s amorphous carbon, where the atoms are connected together all willy-nilly. There’s no real overall structure to the substance.
'Graphite is another form of carbon, where the atoms bond together to form interconnected hexagons in flat sheets (different structures using the same elements are called allotropes). Graphite is surprisingly strong along the plane of each individual sheet, but the sheets themselves slide around on top of each other easily.
'Diamonds are a third form of carbon, where the atoms come together to bond in the form of interconnected tetrahedra (like the corners of a four-sided triangular pyramid). A diamond is essentially a gigantic carbon molecule, and the bonds are strong in all three dimensions. That’s why diamonds are so hard; the bonds are very tough to break. Terrestrial diamonds form when carbon under the Earth’s crust is subjected to high pressure and temperature, allowing the atoms to assemble in the tetrahedral pattern.'