Diamond is a solid form of carbon with its atoms organized in a crystal structure which is called diamond cubic. At room pressure and temperature, another solid form of carbon known as graphite is the chemically stable carbon form, but diamond rarely converts to it. Diamond has the highest thermal conductivity and hardness of any natural material, properties that are used in major industrial applications such as polishing and cutting tools. They are also why diamond anvil cells can subject materials to pressures located deep in the Earth.
The equilibrium temperature and pressure conditions for a transition between diamond and graphite are well established experimentally and theoretically. The pressure changes between 1.7 GPa at 0 K and 12 GPa at 5000 K. However, the phases have a vast region about this line where they can synchronize. At standard pressure and temperature, 293 K (20 °C ) and one standard atmosphere, the stable form of carbon is graphite. Still, diamond is metastable, and its rate of conversion to graphite is rather negligible. However, at heats above about 4500 K, diamond quickly converts to graphite.
Diamonds are scarce, with concentrations of at most parts per billion in the source rock. Before the 20th century CE, humans found most diamonds in alluvial deposits. Loose diamonds are also located along ancient and existing shorelines, where they tend to accumulate because of their density and size.
Almost all diamonds come from the mantle of Earth, and most of this section explains those diamonds. However, there are additional sources. Some crust blocks, or terranes, have been immersed deep enough as the crust thickened, so they encountered ultra-high-pressure metamorphism. These have uniformly distributed microdiamonds that reveal no sign of transportation by magma. Also, when meteorites hit the ground, the shock wave can create high enough pressures and temperatures for nanodiamonds and microdiamonds to form. Impact-type microdiamonds can be utilized as a sign of ancient impact craters.
Diamonds are far from uniformly distributed over the Earth. Clifford’s rule states that they are virtually always found in kimberlites on the oldest cratons, the stable nuclei of continents with typical ages of 2 billion years or more. However, there are genuine exceptions. The Argyle diamond mine in Australia, the biggest producer of diamonds by weight globally, is situated in a mobile belt, also called an orogenic belt, a weaker zone encompassing the central craton with sustained compressional tectonics.
Kimberlite pipes can be challenging to find. They weather quickly and tend to have lower topographic relief than surrounding rock. If they are evident in outcrops, the diamonds are never noticeable because they are extremely rare. In any case, kimberlites are mostly covered with sediments, vegetation, lakes, or soils. In recent searches, geophysical methods such as electrical resistivity, aeromagnetic surveys, and gravimetry help identify likely regions to explore.