It is a common belief that diamonds, with their exceptional brilliance and hardness, are formed from coal deep within the Earth. This widespread notion often suggests that coal, under immense pressure and heat, transforms directly into these valuable gemstones. However, the true geological origins of diamonds are far more complex and distinct from the processes that create coal, revealing a fascinating story of Earth’s dynamic forces.
The Coal Connection Myth
The idea that diamonds originate from coal is a persistent misconception, despite both being carbon-based. Their formation conditions, depths, and carbon sources are fundamentally different. Coal forms from ancient plant matter, while diamonds are much older than Earth’s first land plants, indicating coal rarely plays a role in diamond formation.
Geologically, coal seams are found as horizontal rock units in relatively shallow parts of the Earth’s crust, not exceeding depths of a few kilometers. In contrast, diamonds form at significantly greater depths within the Earth’s mantle. Coal contains various impurities like hydrogen, oxygen, nitrogen, and sulfur, which prevent the pure carbon crystal structure required for diamond formation. Diamonds, conversely, are composed of nearly pure carbon with a highly ordered atomic arrangement.
How Diamonds Actually Form
Diamonds form under extreme conditions of temperature and pressure, found deep within the Earth’s mantle. These conditions include temperatures ranging from approximately 900 to 1,300 degrees Celsius and pressures of 4.5 to 6 gigapascals, which is roughly equivalent to 45,000 to 60,000 times atmospheric pressure. Such immense forces are present at depths between 140 and 190 kilometers (about 90 to 125 miles) below the Earth’s surface, although some diamonds can originate from depths as great as 800 kilometers. This environment is where carbon atoms crystallize into the dense, ordered structure of diamond.
The carbon source for most natural diamonds is carbon trapped within the Earth’s interior since its formation, or from the subduction of oceanic plates. These subducted materials can include carbonate rocks like limestone, marble, and dolomite, which are carried deep into the mantle where their carbon can be incorporated into diamond growth. Once formed, diamonds are brought closer to the Earth’s surface through rare, rapid, and powerful volcanic eruptions. These eruptions create distinctive geological structures known as kimberlite and lamproite pipes, which act as conduits, transporting diamonds from the mantle to accessible depths in the crust. Most natural diamonds are ancient, with ages ranging from 1 billion to 3.5 billion years.
The Origin of Coal
Coal forms through a process called coalification, transforming dead plant matter. This process begins in swampy wetland environments where abundant plant material, such as giant ferns and trees, accumulates faster than it can fully decompose. These low-lying areas allow layers of organic material to build up and convert into peat through microbial activity.
Over geological time, these peat layers become buried under successive layers of sediment and rock. The increasing pressure and moderate heat from this burial gradually compact the peat, squeezing out water and other compounds. This process increases the carbon concentration, leading to the formation of different ranks of coal, from lignite to subbituminous, bituminous, and eventually anthracite. Coal forms at relatively shallow depths within the Earth’s crust, within 3.2 kilometers (about 2 miles), a stark contrast to the deep mantle origins of diamonds.