A diamond is a crystalline form of the element carbon. This solid material is an allotrope of carbon, meaning it is one of several physical forms the element can take. A pure diamond is composed of carbon atoms and is recognized as the hardest naturally occurring substance on Earth. Most natural diamonds are approximately 99.95 percent carbon; the remainder consists of trace elements that can affect the stone’s color.
The Elemental Component: Carbon
The defining characteristic of a diamond is its atomic structure, which is entirely composed of carbon atoms. Unlike softer carbon forms, such as graphite, the atoms in a diamond are arranged in a dense, three-dimensional lattice. Each carbon atom is strongly bonded to four neighboring carbon atoms in a tetrahedral formation.
These connections are strong covalent bonds, creating a rigid and stable crystal structure. This specific arrangement of atoms, known as the diamond cubic crystal structure, is responsible for the material’s extreme density and hardness. The tight atomic packing requires immense energy to break, which is why diamonds are used in cutting tools and are durable as gemstones.
Natural Diamond Formation: Mantle Conditions
The creation of a natural diamond requires a rare combination of heat and pressure deep beneath the Earth’s surface. This process occurs in the Earth’s mantle, far below the crust, in a region geologists call the “diamond stability field.” The necessary conditions are found at depths ranging from about 90 to 150 miles underground.
Temperatures in this deep mantle zone must range between 1,652 and 2,372 degrees Fahrenheit (900°C to 1,300°C). Simultaneously, the carbon-bearing material must be subjected to pressures of approximately 650,000 to 870,000 pounds per square inch. This intense environment forces the carbon atoms to reorganize into the tightly packed lattice that forms the diamond crystal. The formation process is slow, often taking between one billion and 3.3 billion years.
The Ascent: Bringing Diamonds to the Surface
Diamonds are not formed near the Earth’s surface and must be transported quickly to be preserved. If the crystals remained in the upper mantle or ascended slowly, the drop in pressure and temperature would cause them to revert to graphite. The mechanism that brings these treasures to the surface is a rare, violent type of volcanic eruption.
These deep-source eruptions tap into the mantle and create specialized vertical rock formations known as kimberlite and lamproite pipes. The magma in these pipes rises rapidly, acting as a geological elevator for the mantle rock fragments, or xenoliths, that contain diamonds. This swift ascent locks the carbon atoms into the diamond structure by preventing transformation back into a softer form.
Synthesizing Diamonds: Man-Made Processes
Modern technology allows scientists to replicate natural diamond-forming conditions in a controlled laboratory setting. These synthetic diamonds are chemically and structurally identical to their natural counterparts, differing only in their origin. Two primary methods dominate the production of man-made diamonds.
The High-Pressure/High-Temperature (HPHT) method mimics the mantle environment. This process involves placing a small diamond seed crystal into a press with a carbon source, typically graphite, and a metal catalyst. The press applies pressures of 5 to 6 GPa and temperatures up to 2,900°F (1,600°C), causing the carbon to dissolve and crystallize onto the seed.
The second method is Chemical Vapor Deposition (CVD), which operates at lower pressures and temperatures. This process uses a vacuum chamber where a diamond seed is exposed to a carbon-rich gas mixture, such as methane and hydrogen. Energy, often microwave plasma, breaks down the gas, allowing pure carbon atoms to deposit layer by layer onto the seed crystal. The CVD process takes several weeks and operates at temperatures between 1,472°F and 2,192°F (800°C and 1,200°C).