go down into the Earth, yes, the pressure increases, but the temperature also increases, so you would never form stishovite.

Where did this stishovite come from? Meteorite impacts. When a meteorite hits the Earth, such as the one that killed off the dinosaurs 65 million years ago, the high-pressure shock wave of the impact will send quartz right into a stishovite phase. There are many craters around the world that have been identified as being impact craters as opposed to fossil volcanic craters by displaying the presence of stishovite.

[Q: Well, yeah, I'm assuming like as soon as the pressure gets higher like it turns into stishovite, but when pressure gets lower why doesn't it go back to quartz?]

Excellent question. You shock the material, and it became stishovite, but then when the impact is over, why doesn't the stishovite turn right back into quartz?

It turns out that it does turn to quartz, but it does so very slowly, so it continues to exist in a metastable state. It turns out that reactions occur very slowly when materials are cold, and the surface of the Earth is quite cold compared to the Earth's interior. The rate of the reaction, the speed at which this stishovite converts to the quartz, is slow, on the order of tens of billions of years.

Perhaps the best example of this is diamond. Diamonds are not stable at the surface. Don't ever let a diamond get near a flame. It will burn like a piece of coal - a rather expensive source of fuel. The reason it will burn is that it's not stable at these conditions. Diamonds actually form at least 150 kilometers beneath us, which is interesting because if you see a natural diamond at the surface it is a rock that has come up from at least 150 kilometers down. There are only a few places in the world where this process has occured. It just so happens that a lot of these regions are in South Africa, which is why so many of the diamond mines have been there.

But the diamond is the same material as the graphite in pencils ­ it's all carbon atoms. However, graphite involves very weak van der Waals bonds, just like in talc. As a result, the sheets of graphite just peel off from each other. When you compress the diamond to a very high level, however, all of those carbons are in covalent bonds with each other, and they are very tightly bound.

I remember an episode from the old Superman television series. Lois Lane loses her diamond ring in quicksand, and when she's not looking, Clark Kent takes a piece of coal and squeezes it in his hands to an inhumanly high pressure, and recreates her diamond. "Here, Lois, I found your diamond."

This is essentially what the Earth does. It can take carbon in the form of graphite and squeeze it. The atoms rearrange themselves into a much denser structure: diamond. When you take the pressure away, however, (when Clark opens his hands) the diamond will slowly turn back into graphite, but this takes billions of years to do, and so during our lifetimes it's not going to cause any appreciable damage.

(by Michael Wysession )