From a thermodynamic standpoint, any diamond on Earth's surface is chemically unstable. At ambient temperature and pressure, the equilibrium phase of pure carbon is not the precious gem, but humble graphite. If an engagement diamond does not spontaneously degrade into pencil lead, it is because the kinetic energy barrier required to rearrange its crystal lattice is colossal. However, inside the Earth, exposed to the scorching heat of magma, that transformation is lethally fast.
For diamonds forged more than 150 kilometers deep to reach the surface intact, the planet had to devise an extreme transport mechanism: kimberlite eruptions. Unlike the conventional magma of an ordinary volcano, kimberlites are ultralight fluids extraordinarily loaded with water and carbon dioxide in a supercritical state.
As the igneous mass ascends through the lithosphere, the pressure drop triggers explosive gas decompression, similar to shaking and uncorking a colossal geological champagne bottle. In the final stretches of its ascent toward the crust, the gaseous and pyroclastic mixture accelerates drastically, reaching calculated speeds ranging from hundreds of kilometers per hour to up to Mach 3 (three times the speed of sound).
This underground bolide tears fragments of mantle rock away, known as xenoliths, which contain the diamond crystals. The entire journey from the depths of the mantle is completed in just a few hours. The dizzying speed and almost instantaneous cooling caused by expanding gases freeze the diamond's atomic structure, preventing its graphitization and preserving the gem forever.
Source: Nature Geoscience - 'The ascent and eruption of kimberlite magmas'
