The idea that diamonds form from compressed coal deep within the Earth is widely accepted. This common misconception often links the precious gemstone to the fossil fuel, suggesting a shared origin. However, the true story of diamond formation is a distinct geological process, far removed from the conditions that create coal. This article will explore the actual genesis of natural diamonds, the separate formation of coal, and clarify why these two carbon-based substances have entirely different origins.
The Genesis of Natural Diamonds
Natural diamonds originate deep within the Earth’s mantle, under conditions of extreme pressure and high temperature. These environments are typically found at depths of 150 to 200 kilometers (93 to 125 miles) below the surface, specifically beneath ancient, stable continental plates. The pressures at these depths range from 4.5 to 6 gigapascals, which is equivalent to approximately 45,000 to 60,000 times the atmospheric pressure at Earth’s surface. Temperatures in these diamond-forming regions typically fall between 900 and 1,300 degrees Celsius (1,650 to 2,370 degrees Fahrenheit).
The carbon source for these diamonds is not organic matter from the Earth’s surface. Instead, it is primarily ancient carbon that has been trapped within the mantle since the Earth’s formation, or carbon recycled from oceanic crust that has been subducted deep into the mantle. Under these intense conditions, carbon atoms are forced into a compact, tetrahedral crystal structure, which gives diamonds their exceptional hardness. Most natural diamonds discovered today have ages ranging from 1 billion to 3.5 billion years, predating land plants that form coal.
The Formation of Coal
In contrast to diamonds, coal is a sedimentary rock formed from ancient plant matter. This process begins in swampy environments where dead vegetation accumulates in waterlogged conditions, preventing complete decay. Over millions of years, these layers of plant debris transform into peat. As more sediments accumulate, the peat becomes buried deeper, subjecting it to increasing pressure and temperatures.
This transformation, known as coalification, occurs under much lower pressures and temperatures compared to diamond formation. Coal typically forms close to the Earth’s surface, usually within a few kilometers, unlike the deep mantle origins of diamonds. Temperatures involved in coal formation range from about 35 to 245 degrees Celsius (95 to 473 degrees Fahrenheit), depending on the type of coal, and the process takes millions of years.
Dispelling the Myth: Why Coal Isn’t the Source
The notion that diamonds form from coal is a widespread misconception, but geological evidence clearly shows this is not the case. The fundamental difference lies in the vastly different conditions required for their formation and their distinct carbon sources. Diamonds require extreme pressure and high temperatures found only in the deep mantle, while coal forms from plant matter in shallow, low-pressure environments.
Furthermore, the carbon in diamonds is ancient and non-organic, predating the land plants that form coal by billions of years. Therefore, coal is neither the raw material nor formed under the necessary conditions for diamond creation.
Diamond’s Journey to the Surface
Once formed deep within the Earth’s mantle, diamonds must journey to the surface. This dynamic process is primarily facilitated by deep-seated volcanic eruptions. These eruptions occur through geological structures known as kimberlite and, less commonly, lamproite pipes. These pipes rapidly transport diamonds and other mantle rocks from depths of over 150 kilometers to the Earth’s crust.
The ascent through these pipes is crucial for diamonds to maintain their crystalline structure. If transport were slow, diamonds would likely revert to graphite due to changing pressure and temperature conditions closer to the surface. This rapid, often violent, ascent protects the diamonds from being reabsorbed or transformed. Kimberlite and lamproite magmas, rich in volatiles, drive this swift upward movement, allowing diamonds to reach accessible depths where they can eventually be mined.