Four Ways That Diamonds Are Formed 

–Melissa Clason

Melissa graduated from NC State University with a Bachelor’s Degree in geology in 2015 and currently works as a geotechnical lab technician.

Diamonds are probably the most beloved mineral on Earth. These beautiful pieces of pure carbon are useful for so many tasks: their hardness and thermal conductivity make them excellent materials for tools that cut and polish, they can be used in laboratories to contain high-pressure experiments, and their brilliant shine makes them beautiful gemstones.

Diamonds can hold a lot of meaning for a person. Some ancient cultures, particularly early Indian and Chinese cultures, believed that diamonds could ward off evil, and would wear them for protection. The ancient Greeks believed that diamonds were created from Zeus’s divine intervention when a group of young men on Crete disturbed him; diamonds were considered sacred to Zeus and were worn by his devotees as a symbol of piety.

You may be wearing a diamond on your finger. For you its endurance symbolizes the endurance of your love for your spouse. But, where do diamonds come from? How are diamonds made? Well, there are four main processes that control the formation of diamonds, using extreme temperature (over 2,700 degrees Fahrenheit) and pressure (at least 725,000 pounds per square inch, or the pressure of 4,000 grown men standing on your foot).

How Are Diamonds Formed by Deep Source Volcanic Eruptions?
Contrary to popular belief, diamonds do not come from coal, although they may be similar in chemical composition. While coal forms from the decay of plant matter and the subsequent burial and solidification of leftover carbon, the most common way diamonds are formed is through eruptions shooting upward from the mantle in vertical kimberlite pipes. Kimberlite is a blue-tinted, coarse grained rock that was named after Kimberley, the location where the famous Star of South Africa was found. Kimberlites are shot through layers of rock in penetrating, rising veins when an eruption in the mantle happens.

Diamonds need a lot of heat as well as pressure to form, and the mantle has high temperatures and the pressure of about 100 miles of overlying rock. The right conditions are not found everywhere, as the mantle flows and circulates heat, and the overlying rock may vary in thickness and density (oceanic crust will be thinner and more dense than thick, buoyant continental crust).

Most times, diamonds will form underneath the interior region of continental plates, where there is more weight and less movement from tectonic activity. The source of the carbon that becomes diamond is carbon that was trapped deep in the upper mantle or lower crust during the formation of the planet. Fluids flowing through the mantle become pressurized and gain momentum while moving, and eventually erupt, pushing past this trapped carbon and forging it into diamond as the magma makes its way towards the surface. The diamonds are left in vertical Kimberlite conduits that sometimes extend down for miles.

This kind of eruption, even though it’s the most common method of forming diamonds, is rare and has not been seen for at least the past 400 years. This kind of diamond formation can be seen in the US at Crater of Diamonds State Park, where you can mine your own diamonds and take home any nice finds. This is where most African diamonds come from as well.

How Are Diamonds Created by Subduction of Tectonic Plates?
Sometimes, diamonds are formed when parts of the Earth’s crust are subducted (one plate is dragged beneath another when they collide due to composition and density differences. The plate that goes underneath the other one is said to be subducted, and is often destroyed by the heat of the magma or more rarely recycled and pushed back up during volcanic eruptions) and brought back up to the surface, or pressurized and pushed during the collision of two continents.

This method of diamond formation does not require temperatures and pressures that are quite as extreme; they can form as close to the surface as 80 kilometers and at 200 degrees Celsius.

Typically, an oceanic plate will subduct beneath a continent, and so the source of carbon for this diamond formation method comes from things you find at the bottom of the ocean: the calcium carbonate shells of dead marine organisms, decaying plant matter that sunk to the ocean floor, and carbonate rocks like limestone and dolomite that are commonly found in oceanic crust.

Carbon from continental collisions may come from carbonate rocks or mantle carbon being pushed upwards. Many Indian diamonds were created by the continent-continent collision that continues to push the Himalayas higher today, because there are ophiolites (sections of the Earth’s oceanic crust that have been thrust on top of continental crust) in several areas of the Himalayas containing many carbonate rocks and fossils.

How Are Diamonds Forged by Asteroid Impact?
Throughout the 4.6 billion years Earth has been a planet, it has actually been frequently hit by asteroids; the high pressures and temperatures created in these collisions have enabled the creation of diamonds on the Earth’s surface. When the asteroid hits belts of carbonate rock and occasionally coal seams exposed at the surface, it pressure-cooks the carbon into diamond.

This theory of diamond formation has been confirmed by geologists who have found miniature diamonds around the ring of craters left by meteorite impacts. In the US, these can be seen at Meteor Crater in Arizona, with diamonds less than a millimeter in diameter. Russia has the famous Popigai Crater, which has produced 13-millimeter diamonds. The Chicxulub Crater on the Yucatan Peninsula, which may have been the crater left from the asteroid that possibly killed the dinosaurs, contains diamonds that get progressively larger the closer you get to the center of the crater.

How Are Diamonds Crystallized Inside of Meteorites?
Extremely tiny diamonds that are only a couple of nanometers in diameter have been found inside meteorites. These diamonds are thought to have formed during collisions with other meteors, similar to how diamonds form by pressure during asteroid impacts on Earth. The carbon within these meteors typically exists only in trace amounts left over from the formation of the universe, or can be found in the remnants of shattered, ancient planets, but the amount of carbon is substantial enough to take form and it is able to be molded through pressure and heat. The Allende meteorite, for example, contains diamond particles that are older than our solar system.