Diamonds have fascinated humanity for centuries. Their formation involves extreme conditions, both deep within the Earth and in a controlled laboratory setting.
The Extreme Conditions of Natural Diamond Formation
Most natural diamonds originate deep within the Earth’s mantle, 150 to 200 kilometers (90 to 125 miles) below the surface. Here, carbon atoms are subjected to immense pressure, 45 to 60 kilobars (4.5 to 6 gigapascals). Temperatures also soar, reaching 900 to 1,300 degrees Celsius (1,650 to 2,370 degrees Fahrenheit).
These conditions of high pressure and temperature are necessary to force carbon atoms into the tightly packed crystalline structure that defines a diamond. The carbon source can be primordial carbon or recycled carbon from subducted oceanic crust, carbonates, or ancient organic materials carried deep into the mantle.
How Diamonds Travel to the Surface
After their formation in the deep mantle, diamonds are brought closer to the Earth’s surface through deep-seated volcanic eruptions. These eruptions occur via conduits known as kimberlite and lamproite pipes, with kimberlite being the predominant type. These volcanic systems rapidly transport mantle material, including diamonds, to the crust.
The speed of this ascent is crucial for preserving the diamonds; if the journey were too slow, the diamonds would revert to graphite due to decreasing pressure and temperature. Kimberlitic magma, rich in volatile compounds, contributes to the rapid and explosive nature of these eruptions. This allows diamonds to reach accessible depths quickly, maintaining their crystalline structure.
Other Ways Natural Diamonds Form
While mantle formation accounts for most natural diamonds, other less common methods also exist, all involving significant pressure. Some diamonds form during extraterrestrial impacts, such as when large meteorites strike the Earth. The extreme shockwaves generated by these collisions create intense pressures and temperatures capable of transforming carbon in the target rock into small diamonds.
Diamonds can also form in subduction zones, where oceanic plates are forced beneath continental plates. As carbon-rich sediments are carried deep into the mantle, they experience increasing pressure and temperature. These diamonds typically form at shallower depths and lower temperatures than those from the mantle source, but still under conditions of high pressure.
Making Diamonds in a Lab
Diamonds are also created in laboratories, and these processes involve the application of pressure. The High-Pressure/High-Temperature (HPHT) method mimics the natural conditions found deep within the Earth. Carbon material, often graphite, is subjected to 5 to 6 gigapascals of pressure and temperatures between 1,300 and 1,600 degrees Celsius, often with a metal catalyst, to grow diamonds.
Chemical Vapor Deposition (CVD) is another common laboratory method. This process involves placing a diamond seed crystal in a vacuum chamber filled with carbon-containing gases, such as methane and hydrogen. These gases are heated to 700 to 1,200 degrees Celsius, causing carbon atoms to deposit layer by layer onto the seed, forming a diamond. Although CVD operates at lower pressures than HPHT, it still relies on controlled conditions to arrange carbon atoms. Lab-grown diamonds are chemically, physically, and optically identical to their natural counterparts.