The origins of diamonds are as fascinating as their beauty. Diamonds are formed deep within the Earth, about 100 miles below ground level in a section of our planet's core known as the upper mantle. The combination of high temperature and high pressure is what's necessary to naturally grow diamond crystals, and as far as we know, all diamonds that formed in the Earth did so under those conditions. This portion of the mantle, however, is too deep for drills to reach. The diamonds that we see nearer to the surface are ones then that were brought up by very violent volcanic eruptions. Most of these probably occurred at a time when the Earth was hotter, explaining why they were more deeply rooted than the eruptions we see in modern times. These eruptions carried the already-formed diamonds from the upper mantle to the surface of the Earth.
The lava flow built up a mound of volcanic material that eventually cooled, and the today's obtainable diamonds are contained within that layer. These are the so-called Kimberlites that are typically the sources of many of the world's mined diamonds.Diamonds are made of carbon atoms that have bonded together to start growing crystals. Because of the temperature and pressure during formation, carbon atoms will attached to each other in a very strong type of bond where each carbon atom is connected to four others. Diamonds are the result of a process of atoms locking together in a repeating network; each of these crystals represents billions and billions of carbon atoms locked together. It is the inherent strength of carbon itself -- and these bonds -- that makes diamond such an impermeable substance.