How are diamonds formed?

Diamonds are formed deep within the Earth about 100 miles or so below the surface in the upper mantle. Obviously in that part of the Earth it's very hot. There's a lot of pressure, the weight of the overlying rock bearing down, so that combination of high temperature and high pressure is what's necessary to grow diamond crystals in the Earth. As far as we know, all diamonds that formed in the Earth formed under those kinds of conditions and, of course, that's a part of the Earth we can't directly sample. We don't have any way of drilling to that depth or any other way of traveling down to the upper mantle of the Earth.

How do diamonds travel to the surface of the Earth?

The diamonds that we see at the surface are ones then that are brought to the surface by a very deep-seated volcanic eruption. It's a very special kind of eruption, thought to be quite violent, that occurred a long time ago in the Earth's history. We haven't seen such eruptions in recent times. They were probably at a time when the earth was hotter, and that's probably why those eruptions were more deeply rooted. These eruptions then carried the already-formed diamonds from the upper mantle to the surface of the Earth. When the eruption reached the surface it built up a mound of volcanic material that eventually cooled, and the diamonds are contained within that. These are the so-called Kimberlites that are typically the sources of many of the world's mined diamonds.
Schematic diagram of a volcanic pipe

Diamond-bearing rock is carried from the mantle to the Earth's surface by deep-origin volcanic eruptions. The magma for such a volcano must originate at a depth where diamonds can be formed—150 km (93 mi) or more (three times or more the depth of source magma for most volcanoes). This is a relatively rare occurrence. These typically small surface volcanic craters extend downward in formations known as volcanic pipes. The pipes contain material that was transported toward the surface by volcanic action, but was not ejected before the volcanic activity ceased. During eruption these pipes are open to the surface, resulting in open circulation; many xenoliths of surface rock and even wood and fossils are found in volcanic pipes. Diamond-bearing volcanic pipes are closely related to the oldest, coolest regions of continental crust (cratons). This is because cratons are very thick, and their lithospheric mantle extends to great enough depth that diamonds are stable. Not all pipes contain diamonds, and even fewer contain enough diamonds to make mining economically viable. Diamonds are very rare because most of the crust is too thin to permit diamond crystallization, whereas most of the mantle has relatively little carbon.

The magma in volcanic pipes is usually one of two characteristic types, which cool into igneous rock known as either kimberlite or lamproite. The magma itself does not contain diamond; instead, it acts as an elevator that carries deep-formed rocks (xenoliths), minerals (xenocrysts), and fluids upward. These rocks are characteristically rich in magnesium-bearing olivine, pyroxene, and amphibole minerals which are often altered to serpentine by heat and fluids during and after eruption. Certain indicator minerals typically occur within diamantiferous kimberlites and are used as mineralogical tracers by prospectors, who follow the indicator trail back to the volcanic pipe which may contain diamonds. These minerals are rich in chromium (Cr) or titanium (Ti), elements which impart bright colors to the minerals. The most common indicator minerals are chromium garnets (usually bright red chromium-pyrope, and occasionally green ugrandite-series garnets), eclogitic garnets, orange titanium-pyrope, red high-chromium spinels, dark chromite, bright green chromium-diopside, glassy green olivine, black picroilmenite, and magnetite. Kimberlite deposits are known as blue ground for the deeper serpentinized part of the deposits, or as yellow ground for the near surface smectite clay and carbonate weathered and oxidized portion.
Looking for diamonds in the river

Once diamonds have been transported to the surface by magma in a volcanic pipe, they may erode out and be distributed over a large area. A volcanic pipe containing diamonds is known as a primary source of diamonds. Secondary sources of diamonds include all areas where a significant number of diamonds have been eroded out of their kimberlite or lamproite matrix, and accumulated because of water or wind action. These include alluvial deposits and deposits along existing and ancient shorelines, where loose diamonds tend to accumulate because of their size and density. Diamonds have also rarely been found in deposits left behind by glaciers (notably in Wisconsin and Indiana); in contrast to alluvial deposits, glacial deposits are minor and are therefore not viable commercial sources of diamond.
What is carbon's role in forming diamonds?

Diamonds are made of carbon so they form as carbon atoms under a high temperature and pressure; they bond together to start growing crystals. Because of the temperature and pressure, under these conditions, carbon atoms will bond to each other in this very strong type of bonding where each carbon atom is bonded to four other carbon atoms. That's why a diamond is such a hard material because you have each carbon atom participating in four of these very strong covalent bonds that form between carbon atoms. So as a result you get this hard material. Again where the carbon is coming from, how quickly they're growing, those are all still open questions, but obviously the conditions are such that you've got some group of carbon atoms that are in close enough proximity that they start to bond. As other carbon atoms move into the vicinity they will attach on. That's the way any crystal grows. It's the process of atoms locking into place that produces this repeating network, this structure of carbon atoms, that eventually grows large enough that it produces crystals that we can see. Each of these crystals, each diamond, one carat diamond, represents literally billions and billions of carbon atoms that all had to lock into place to form this very orderly crystalline structure.
The Cullinan Diamond is 3,106,75 carats (621.35 grams) diamond and is the largest rough gem quality diamond ever found in the world.

References: 1, 2, 3, 

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