Lab-grown diamonds are synthesized in controlled environments, replicating the atomic structure of those found in nature. They possess the exact same chemical composition, crystal structure, and optical properties as their natural counterparts. Scientists achieve this by accelerating the natural diamond formation process using two primary methods: the High-Pressure/High-Temperature (HPHT) method and the Chemical Vapor Deposition (CVD) method.
Synthesis Through Extreme Pressure and Heat
The High-Pressure/High-Temperature (HPHT) method mimics the conditions deep within the Earth’s mantle where natural diamonds form. This process requires specialized apparatus capable of maintaining immense forces and intense heat for several weeks. The synthesis is carried out inside massive mechanical presses designed to generate and sustain these extreme conditions.
Inside the press, a small diamond seed crystal is placed at the bottom of a growth cell containing a carbon source, typically high-purity graphite. This assembly is subjected to temperatures ranging from 1,300 to 1,600 degrees Celsius. Simultaneously, a tremendous pressure is applied, reaching approximately 5.5 GigaPascals (GPa), equivalent to 55,000 times the atmospheric pressure at sea level.
A metal solvent-catalyst, commonly an alloy containing iron, nickel, or cobalt, is also included in the growth cell. This alloy lowers the required pressure and temperature conditions to a range achievable in the laboratory. Under the extreme heat, the metal melts and dissolves the carbon source material.
The dissolved carbon then migrates through the molten metal flux toward the cooler diamond seed crystal. As the carbon atoms reach the seed, they precipitate out of the solvent and crystallize onto the existing structure. This controlled recrystallization process builds the new diamond structure layer by layer.
Synthesis Through Gas Deposition
The Chemical Vapor Deposition (CVD) method utilizes a low-pressure environment instead of the intense force of the HPHT process. This technique involves growing diamonds inside a vacuum chamber. CVD operates at significantly lower pressures, typically sub-atmospheric, and temperatures generally ranging between 700 and 1,200 degrees Celsius.
A thin, flat diamond seed crystal is placed onto a substrate holder within the vacuum chamber. The chamber is then filled with a carbon-containing gas mixture, commonly composed of methane and hydrogen. Microwave energy is directed into the chamber, which energizes the gases and generates a plasma cloud.
The intense energy from the plasma breaks down the molecular bonds of the hydrocarbon gases, releasing individual carbon atoms. These freed carbon atoms then systematically rain down and deposit onto the diamond seed crystal, where they bond to the existing carbon lattice. The diamond crystal grows slowly, atom by atom, in distinct layers.
CVD crystals often have a brownish tint, caused by the incorporation of nitrogen or other point defects during the growth phase. To improve the color and clarity of these diamonds, they are frequently subjected to a post-growth treatment. This involves placing the CVD-grown diamond into an HPHT press, where extreme heat and pressure alter the defects to produce a colorless or near-colorless gem.
Distinguishing Characteristics of Lab-Grown Diamonds
The process of creating lab-grown diamonds leaves minute, detectable physical evidence. Gemological laboratories utilize specialized equipment to identify these microscopic markers, as the specific growth environment of HPHT and CVD produces different internal features.
HPHT diamonds, grown from a central point outward, often exhibit cuboctahedral or geometric growth patterns inside the crystal structure. They may also contain tiny metallic inclusions, which are microscopic remnants of the iron, nickel, or cobalt solvent-catalyst. These metallic particles can be detected because they are attracted to a strong magnet.
CVD diamonds typically show parallel striations or bands that reflect the horizontal growth planes. They generally lack metallic inclusions but may contain non-metallic inclusions, such as dark graphite. Under short-wave ultraviolet (UV) light, lab-grown diamonds display characteristic fluorescence and phosphorescence patterns.
HPHT diamonds often show a cross-shaped or blocky pattern of fluorescence, while CVD diamonds may exhibit a striped or linear pattern when they fluoresce. Advanced techniques, such as photoluminescence and UV/Vis spectroscopy, analyze trace elements and structural defects. This provides the definitive evidence needed to confirm the diamond’s synthetic origin and the specific method of its laboratory growth.