It's carbon, under extreme pressure. Of course it's common.
DeBeers is a scam.
Formation and growth
Diamonds in the mantle form through a metasomatic process where a C-O-H-N-S fluid or melt dissolves minerals in a rock and replaces them with new minerals. (The vague term C-O-H-N-S is commonly used because the exact composition is not known.) Diamonds form from this fluid either by reduction of oxidized carbon (e.g., CO2 or CO3) or oxidation of a reduced phase such as methane.[12]
Using probes such as polarized light, photoluminescence and cathodoluminescence, a series of growth zones can be identified in diamonds. The characteristic pattern in diamonds from the lithosphere involves a nearly concentric series of zones with very thin oscillations in luminescence and alternating episodes where the carbon is resorbed by the fluid and then grown again. Diamonds from below the lithosphere have a more irregular, almost polycrystalline texture, reflecting the higher temperatures and pressures as well as the transport of the diamonds by convection.[14]
Transport to the surface
Geological evidence supports a model in which kimberlite magma rose at 4–20 meters per second, creating an upward path by hydraulic fracturing of the rock. As the pressure decreases, a vapor phase exsolves from the magma, and this helps to keep the magma fluid. At the surface, the initial eruption explodes out through fissures at high speeds (over 200 meters per second). Then, at lower pressures, the rock is eroded, forming a pipe and producing fragmented rock (breccia). As the eruption wanes, there is pyroclastic phase and then metamorphism and hydration produces serpentinites.[14]
In space
(main article: Extraterrestrial diamonds)
Although diamonds on Earth are rare, they are very common in space. In meteorites, about 3 percent of the carbon is in the form of nanodiamonds, having diameters of a few nanometers. Sufficiently small diamonds can form in the cold of space because their lower surface energy makes them more stable than graphite.
The isotopic signatures of some nanodiamonds indicate they were formed outside the Solar System in stars.[24]
High pressure experiments predict that large quantities of diamonds condense from methane into a “diamond rain” on the ice giant planets Uranus and Neptune.[25][26][27] Some extrasolar planets may be almost entirely composed of diamond.[28]
Diamonds may exist in carbon-rich stars, particularly white dwarfs. One theory for the origin of carbonado, the toughest form of diamond, is that it originated in a white dwarf or supernova.[29][30] Diamonds formed in stars may have been the first minerals.[31]
https://en.wikipedia.org/wiki/Diamond#Formation_and_growth
True! Diamonds can be manufactured today. Not only has DeBeers convinced most world governments that only their diamonds can be legally traded (for the most part), but they have also convinced the world to pass laws requiring "manufactured diamonds" to be marked and devalued.
The reason "blood diamonds" exists is because DeBeers does not own the mines, essentially.
+1 for the Laz :-)