Lab-grown diamonds are chemically and structurally identical to those formed naturally beneath the Earth, consisting of pure carbon atoms arranged in a crystal lattice structure. They are genuine diamonds, not simulants like cubic zirconia, sharing the same physical and optical properties as their mined counterparts. These laboratory-created gems have disrupted the jewelry market, offering a more affordable and popular alternative to traditional diamonds. Their creation is achieved through two distinct, technology-driven processes that replicate natural diamond formation: High-Pressure, High-Temperature (HPHT) and Chemical Vapor Deposition (CVD).
The Foundation: Seed Crystals and Carbon Sources
The starting point for any lab-grown diamond is a tiny diamond slice known as a seed crystal. This high-quality piece acts as a template, providing the correct atomic arrangement onto which new carbon atoms can bond and grow. The seed is typically a thin wafer, often taken from an HPHT-grown diamond, and is cleaned before the growth process begins.
The second requirement is a source of pure carbon to supply the building blocks. For the HPHT method, the carbon source is highly purified graphite. In contrast, the CVD method utilizes a carbon-rich gas mixture, most commonly methane mixed with hydrogen gas.
High-Pressure, High-Temperature (HPHT) Growth
The HPHT method directly mimics the extreme geological conditions deep within the Earth’s mantle where natural diamonds form. This process requires a specialized machine called a press, with three common designs being the belt, cubic, and split-sphere presses.
Inside the press, a growth cell containing the diamond seed, the graphite carbon source, and a metal solvent-catalyst is assembled. The cell is subjected to immense pressure, typically 5 to 6 gigapascals (GPa), and temperatures between 1,300 and 1,600 degrees Celsius. This combination is necessary to push the graphite into the diamond stability field.
A metal alloy flux, often containing iron, nickel, or cobalt, melts and acts as a solvent. This molten metal dissolves carbon from the graphite source in a hotter zone and transports it to the cooler diamond seed crystal. The dissolved carbon atoms precipitate onto the seed, crystallizing layer by layer to form a larger rough diamond. This growth process takes several weeks to complete.
Chemical Vapor Deposition (CVD) Growth
The CVD method grows diamonds in a vacuum chamber at much lower pressures and temperatures than HPHT. The process begins by placing the diamond seed onto a substrate within a sealed chamber. The chamber is evacuated and filled with hydrocarbon gases, primarily methane and hydrogen.
The chamber is heated to 700 to 1,200 degrees Celsius, and microwave energy generates a plasma ball. This plasma breaks down the molecular bonds of the gases, releasing pure carbon atoms. These carbon atoms deposit onto the diamond seed crystal, bonding and slowly building up the structure layer by layer.
This deposition allows for precise control over the diamond’s growth and purity. The CVD process operates at pressures significantly lower than HPHT, and the growth period typically lasts several weeks.
Post-Growth Enhancement and Finishing
Once the rough crystal is removed from the growth chamber, it often undergoes further treatment to optimize its appearance before being cut. A post-growth process called annealing, involving controlled heat treatment, is commonly applied to diamonds from both methods.
This treatment is particularly common for CVD diamonds, which often exhibit a brown or grayish tint due to structural defects created during their growth. Annealing exposes the diamond to high temperatures to rearrange the atomic structure and remove undesirable color centers.
For instance, a secondary HPHT treatment above 1,500 degrees Celsius effectively eliminates the brown hue in CVD stones, creating colorless diamonds. The final steps involve the traditional processes of cutting, shaping, and polishing the rough crystal into a finished, faceted gemstone.
Comparing the Processes and Products
The fundamental difference between HPHT and CVD lies in the conditions and mechanism of crystal formation, leading to distinct structural characteristics. HPHT diamonds grow in a cuboctahedron shape with multiple growth directions, resulting in complex growth patterns and often metallic inclusions from the solvent-catalyst. Conversely, CVD diamonds grow in a cubic shape with growth occurring in a single direction, which can sometimes result in more noticeable internal strain under magnification.
These differences also impact the typical color profiles of the as-grown crystals. HPHT diamonds often incorporate nitrogen, leading to Type Ib characteristics, which typically present with a yellow or brownish tint. The CVD process results in diamonds with very low nitrogen content, making them Type IIa diamonds.
Experts can use specialized equipment to distinguish the origin of a lab-grown diamond by observing these unique growth patterns, internal inclusions, and fluorescence characteristics.