^ Well considering you can make diamonds literally from scratch...
This occurs in reality. A number of fairly hefty gemstones have been unearthed before. Likewise the crystal caves like that mentioned in Mexico.
There are a few theories/hypotheses that what was found in Mexico is a lot more common than thought, particularly in geologically active regions (mountain ranges, volcanic areas, plate boundaries).
Also this has occurred in space. One recently discovered exo-planet is believed to be a giant diamond. No not covered in diamonds, the whole planet is one diamond. (This one.
edited 5th Oct '12 7:55:08 PM by MajorTom