There are four main processes that are believed to be responsible for nearly all diamond deposit sites found near the earth’s surface – diamond formation in the earth’s mantle, diamond formation in a subduction zone, diamond formation at impact sites, and diamond formation in space.
Carbon is one of the most important elements on our planet, which led the Geological Society of London to name 2019 the Year of Carbon. Diamonds are a main host for carbon in the deep earth and also have a deeper origin than all other gemstones. Whereas ruby, sapphire, and emerald form in the earth’s crust, diamonds form many hundreds of kilometers deep in the earth’s mantle.
Another way diamonds are formed is in what's called a subduction zone, which occurs when two tectonic plates collide, and one is forced down into the earth's mantle. When carbon rocks from the surface are subjected to the increasing heat of the mantle combined with the pressure from the colliding plates, tiny diamonds are formed in these rocks.
This process can occur at slightly lower temperatures and shallower depths than those necessary for diamond formation in the "diamond stability zones." When the subducted rocks return to the crust, these small diamonds can be found within them. Subduction-zone diamonds' small size and lack of clarity make them unsuitable for industrial or commercial use.
Throughout its history, Earth has been repeatedly hit by large asteroids. When these asteroids strike the earth, extreme temperatures and pressures are produced. For example: when a six mile (10 kilometer) wide asteroid strikes the earth, it can be traveling at up to 9 to 12 miles per second (15 to 20 kilometers per second). Upon impact this hypervelocity object would produce an energy burst equivalent to many nuclear weapons and temperatures hotter than the sun's surface.
NASA researchers have detected large numbers of nanodiamonds in some meteorites. (Nanodiamonds are diamonds that are a few nanometers - billionths of a meter in diameter.) About three percent of the carbon in these meteorites is contained in the form of nanodiamonds. These diamonds are too small for use as gems or industrial abrasives; however, they are a source of diamond material . See Location 4 in the diagram at the top of the page.