Natural diamond is formed where carbon has crystallized under exposure to high pressure and temperature. The pressure must be between 45 and 60 kilobars and the temperature between 900 and 1300 °C.
These conditions occur naturally only in the lithospheric mantle, below the continental plates, and at meteorite strike sites.
In the lithospheric mantle, the proper temperature and pressure are usually found in depths of 140-190 kilometers. The correct combination of temperature and pressure is only found in the thick, ancient, and stable parts of continental plates where regions of lithosphere known as cratons exist. Presence in the cratonic lithosphere for long periods of time allows diamond crystals to grow larger.
The slightly misshapen octahedral shape of rough diamond crystal in matrix is typical of the mineral. Its lustrous faces also indicate that this crystal is from a primary deposit.
Through studies of carbon isotope ratios (similar to the methodology used in carbon dating, except with the stable isotopes C-12 and C-13), it has been shown that the carbon found in diamonds comes from both inorganic and organic sources. Some diamonds, known as harzburgitic, are formed from inorganic carbon originally found deep in the Earth's mantle. In contrast, eclogitic diamonds contain organic carbon from organic detritus that has been pushed down from the surface of the Earth's crust through subduction before transforming into diamond. These two different source carbons have measurably different 13C:12C ratios. Diamonds that have come to the Earth's surface are generally very old, ranging from under 1 billion to 3.3 billion years old.
The high pressure and temperature required for diamond formation also occur during meteorite impact. Tiny diamonds, known as microdiamonds or nanodiamonds, have been found in meteorite impact craters. These can be used as one indicator of ancient impact craters.
Diamonds formed in extraterrestrial space, then deposited on earth by meteorites, have been found in South America and Africa.
Diamonds are usually brought to the Earth's surface or closer to it by volcanic action and dispersed in an area by water erosion or the action of glaciers. The latter are usually not in high enough concentrations to make them commercially viable sources of diamonds.
Volcanic pipes that reach 150 km or more are relatively rare, but they are the ancient conduits of magma that transported diamonds closer to the surface, where they can be mined.
Certain minerals which are formed and transported from the depths in the same conditions as diamonds, are used as indicators by prospectors looking for sources of diamonds. The most common ones are chromian garnets (usually bright red Cr-pyrope, and occasionally green ugrandite-series garnets), eclogitic garnets, orange Ti-pyrope, red high-Cr spinels, dark chromite, bright green Cr-diopside, glassy green olivine, black picroilmenite, and magnetite.
Every natural diamond is immensely old, formed long before dinosaurs roamed the earth. The youngest diamond is 900 million years old, and the oldest is 3.2 billion years old.