A diamond is an allotrope of carbon. Chemically, it is composed of nearly pure carbon atoms arranged in a specific, isometric crystal structure. The strong covalent bonds form a rigid, tetrahedral network that grants the material its unique properties. This transformation from common carbon to diamond only occurs under extreme conditions of high temperature and intense pressure. The formation of diamonds can happen through natural geological processes, violent extraterrestrial impacts, or highly controlled laboratory manufacturing.
Natural Formation Deep Within Earth
The vast majority of natural diamonds originate deep within the Earth’s mantle, far below the crust in stable continental regions called cratons. The conditions necessary for carbon to crystallize into diamond exist within a narrow stability zone, typically between 90 and 150 miles (150 to 240 kilometers) below the surface.
In this region, the carbon is subjected to pressure ranging from 4.5 to 6 gigapascals (GPa), or approximately 45,000 to 60,000 times the atmospheric pressure at sea level. Temperatures in the diamond stability zone range between 900°C and 1,300°C. This combination of heat and pressure forces the carbon atoms into the dense, tightly bonded crystalline lattice.
The carbon source comes from two main origins: primordial carbon left over from Earth’s formation, and carbon-bearing materials that were subducted into the mantle via plate tectonics. Diamonds remain stable in the cratonic mantle for billions of years until a geological event brings them to the surface.
This transport is accomplished by rare, deep-source volcanic eruptions that create narrow vertical conduits known as kimberlite and lamproite pipes. The ascent must be extremely rapid to prevent the diamonds from reverting back to graphite, which is the stable form of carbon at lower pressures. This swift, explosive transport preserves the diamond’s structure, embedding the crystals in the igneous rock of the pipes where they are later mined.
Formation from Impact Events
A second, less common natural process for diamond creation occurs during massive, high-velocity meteorite or asteroid impacts on Earth’s surface. When a large carbon-containing body strikes the planet, the collision generates an extreme shock wave. This shock compression creates the intense pressures and high temperatures required for diamond formation, though on a nanoscale and nanosecond timescale.
The resulting diamonds are often microscopic. These impact events are also responsible for the formation of lonsdaleite, a rare hexagonal form of carbon that is sometimes found alongside the cubic nanodiamonds. Lonsdaleite is created when graphite-bearing materials are subjected to the massive pressures of the impact, exceeding 170 GPa in some cases.
Evidence of this shock-induced formation process has been found at impact sites like Meteor Crater in Arizona, where the Canyon Diablo meteorite struck. Tiny extraterrestrial diamonds have also been discovered embedded within certain types of meteorites. These meteoritic diamonds formed either during an impact on a planetary body in space or even earlier, as stardust in the cosmos.
Synthetic Diamond Creation
Diamonds are grown in a controlled laboratory environment using two primary methods. The first, High-Pressure/High-Temperature (HPHT), directly mimics the deep-earth formation process.
High-Pressure/High-Temperature (HPHT)
In the HPHT process, a small diamond seed crystal is placed in a growth cell with a carbon source, such as graphite, and a metal flux solvent, typically a mixture of iron, nickel, or cobalt. This assembly is placed inside a massive press, which subjects the materials to pressures of approximately 5–6 GPa and temperatures between 1,300°C and 1,600°C.
Under these conditions, the metal flux melts and dissolves the carbon source. The dissolved carbon then migrates through the molten metal to the cooler diamond seed, where it crystallizes layer by layer to form a larger diamond crystal.
Chemical Vapor Deposition (CVD)
The second major method is Chemical Vapor Deposition (CVD). In the CVD process, diamond seeds are placed inside a vacuum chamber and heated to temperatures between 700°C and 1,200°C.
A small amount of carbon-containing gas, commonly methane, is introduced into the chamber. Energy is used to break down the hydrocarbon gas molecules into a plasma. This plasma releases carbon atoms, which then deposit onto the diamond seed substrate. The CVD method allows for diamond growth at lower pressures compared to HPHT, and it is capable of producing high-purity diamonds, including the rare Type IIa variety.
Identifying Differences Between Diamonds
The differences in formation processes leave behind characteristic, detectable features. Natural diamonds often exhibit irregular, ring-like growth structures and contain mineral inclusions from the mantle rock. They also display distinctive, irregular strain patterns when viewed under polarized light.
HPHT diamonds frequently contain tiny, dark metallic flux inclusions. These diamonds also show blocky growth sectors and a cross-like strain pattern in subdued colors under magnification. The rapid, layered growth of CVD diamonds often results in lamellar growth structures and may show patchy, grainy birefringence patterns.
Specialized gemological instruments detect these differences:
- Spectroscopy detects trace elements characteristic of each formation environment. For example, HPHT diamonds may show absorption lines related to nickel from the metal flux, while CVD diamonds may show a characteristic peak associated with silicon impurities.
- Fluorescence under ultraviolet light is also used, as the reaction patterns and colors can vary significantly between natural and lab-grown stones.