Diamonds are interesting because they are not stable at room temperature. What? Yep. Diamond and graphite are two ways to arrange carbon molecules, and graphite is the more stable of the two. [You can actually measure this by burning a diamond in oxygen and measuring the amount of heat that is released, then comparing that to the amount of heat released by burning the same amount of graphite. The diamond releases slightly more energy when burned. You can think of the carbon-carbon bonds in a diamond as being compressed and twisted away from their preferred orientation, so when the crystal burns, it first untwists, then oxygen atoms come in and react with the carbon. But, I digress.]
So, why is that interesting? It’s because – at atmospheric pressure – carbon will convert to the stable form: graphite. Leave a diamond on a table for a gazillion years, and you’ll find a lump of graphite. More practically, if you heat it to modest temperatures (i.e. 1300C), that change will happen rapidly (i.e. about an hour).
Diamond is stable at high pressures. And, deep (140km) underground, there is enough pressure to squeeze the carbon atoms closer together to make the diamond lattice. No problem. But, how do you get the diamond up to the surface? Not slowly! If you imagine a diamond rising to the surface in a flow of liquid magma, the pressure decreases as it gets close to the surface, but the temperature stays high. Once it’s less than 140km deep, the pressure is low enough so that graphite becomes the stable phase, and the crystal is still hot. An hour or so later, your diamond will be gone, converted back to graphite.
So, those diamonds must come up at pretty respectable speeds: at least 140km/hour, and they come up from the very bottom of the thickest continental crust, far deeper than normal volcanoes. These are not your garden variety eruption. [And the neat thing is that we can figure this out from laboratory experiments.] There hasn’t been one of these eruptions in the last 100,000 years, but it’s generally believed that they are driven by magma that contains a lot of water and gas, and the gas and steam expands, pushing the drops and fragments of magma upwards like shotgun pellets in a gun barrel. The stuff (and diamonds) comes roaring out of the crack at supersonic speeds and spreads out too widely to build a proper mountain. It would be quite a show.
- (See photo of the conical mine about half-way down this page: http://indrus.in/articles/2010/10/11/diamond_miner_braces_for_a_sparkling_makeover04774.html )