Diamond gets its name from the Greek "adamas," the adamant, or unbreakable. A genuine diamond is the hardest of all known natural materials. The only reason diamonds can be cut into such brilliant shapes at all is because of their crystalline structure, which creates natural fracture lines.
The conditions which can build such a material are among the most extreme on Earth. Diamonds cannot form at a pressure of less than 45 kilobars or outside a narrow temperature range of 1600 to 2300 degrees Fahrenheit. These conditions create kimberlite and lamproite, the only kinds of magma which are known to contain diamonds.
The temperatures required for diamond formation are low compared to typical temperatures at these kinds of pressures. They can only be found in the most stable parts of continental plates at depths between 87 and 186 miles, where the Earth is heated only by natural internal radioactivity and not additionally by the pressure of tectonic plate against plate. In general, kimberlite forms in the upper mantle of the lithosphere, while lamproite is believed to form in isolated transition zone melts, as pockets of subducted lithosphere at the base of the lithospheric mantle.
Thus, the earth process of plate tectonics is involved in creating diamonds, but only because it creates highly stable sections of continental plates as well as the geologically dynamic edges. These stable sections are known as cratons.
Diamonds consist of carbon which has seeped out of carbonite-heavy magma and crystallized. Because cratons have survived intact for billions of years, the fledgling diamond crystals they contained were able to grow to the sizes known today. Most diamonds have been carbon dated from 1.2 to 3.3 billion years old.
If diamonds remained where they were originally formed, people could not have found them using current technology. Vulcanism is the earth process which carries diamonds from the mantle to Earth's surface.
Kimberlite and lamproite pipes are unusual because they started as volcanoes which reached down deep into the mantle, roughly 3 times as deep as other types of volcanoes. At the same time, they are also among the thinnest known volcanic pipes, with some only a few dozen yards across. The combination of depth and unique magma type creates characteristic carrot-shaped pipes.
Carbonite-heavy kimberlite rose towards the crust because of the internal pressures. However, once it reached the crust, it was forced suddenly to the surface by the interaction of the carbonite with surrounding silica, when carbon dioxide bubbled out of the rock and created a fast-expanding foam. The more the magma rose, the more silica it encountered, and the more foam was created.
After the magma blew out at the surface, it began at once to cool. After the eruption was over, all that remained was the carrot-shaped kimberlite pipe. Lamproite pipes are wider at the top, more like shallow champagne glasses than carrots, because lamproite also contains corrosive materials which dissolved the surrounding rock.
The earliest discoveries of diamonds were in and around kimberlite pipes which had been uncovered and worn down by erosion. The volcanic pipe itself is the primary source of diamonds.
However, diamonds which have been eroded out of the kimberlite pipe usually end up downstream in alluvial deposits, riverbeds, and sometimes even glacial deposits. All these sources of diamonds are secondary sources.