Robert Gentry: Granite as a "Genesis Rock"

Similarly, diamonds can't be made by crystallizing molten carbon at atmospheric pressure. This, however, doesn't put all the people that sell synthetic diamonds out of business - they just had to adjust the conditions of pressure and temperature to those where diamond crystals can grow. Granite will form from a melt if the pressure and temperature are right, and if enough time is allowed for crystallization to proceed. It may well be that granite hasn't been made in the laboratory because that time is longer than the amount of time you can keep a pressurized oven at the right numbers, or that the time is longer than a grad student will stay in school. Granite is forming every day down under some volcano somewhere........