A widespread belief suggests that diamonds form from coal, with immense pressure and heat transforming coal into glittering gemstones. This article clarifies the actual scientific processes behind diamond formation, distinguishing it from how coal comes into being.
Unraveling the Myth
The notion that coal transforms into diamonds is largely inaccurate for most natural diamonds. Coal is a sedimentary rock formed from ancient plant matter, such as trees and ferns, which accumulated in swampy environments over millions of years. This organic material is buried under layers of sediment, undergoing coalification where increasing temperature and pressure compact and alter it into coal. Coal deposits are typically found relatively close to the Earth’s surface, generally within a few kilometers deep. Even the highest-grade coal forms at temperatures around 180-245°C, usually at depths of a few kilometers, significantly less than what is needed for diamond formation.
Diamonds, on the other hand, are pure carbon crystals that form under vastly different and more extreme conditions. They require immense pressure, typically ranging from 4.5 to 6 gigapascals (approximately 45,000 to 60,000 times atmospheric pressure), and high temperatures, usually between 900 to 1,300 degrees Celsius. These conditions are found deep within the Earth’s mantle, at depths of about 140 to 200 kilometers below the surface. The carbon source for most natural diamonds is not coal, but rather carbon trapped within the Earth’s mantle during its formation or carbon recycled from the Earth’s surface through the subduction of oceanic crust. This fundamental difference in formation environments and carbon sources means that coal rarely, if ever, serves as the precursor for natural diamonds.
The True Story of Diamond Formation
Natural diamonds originate deep within the Earth’s mantle, in ancient, stable parts of continental plates known as cratonic roots. Here, extreme pressure and temperature allow carbon atoms to bond in a unique crystal lattice structure, giving diamonds their exceptional hardness and brilliance. This crystallization occurs over millions to billions of years. The carbon atoms involved are primordial carbon from the Earth’s early formation or carbon carried into the mantle through geological processes like plate subduction.
Once formed, diamonds remain deep within the mantle until geological forces bring them closer to the surface. This transport occurs through rare, powerful volcanic eruptions that create kimberlite and lamproite pipes, vertical conduits extending from the deep mantle to the Earth’s crust. Magma within these pipes ascends rapidly, carrying diamonds and other mantle rocks towards the surface. This swift ascent is crucial, as it prevents diamonds from reverting to graphite, a more stable form of carbon at lower pressures. These ancient volcanic events are responsible for delivering most natural diamonds to accessible depths.