How Are Lab Grown Diamonds Made: A Step-by-Step Guide to CVD and HPHT Processes

Lab grown diamonds are made through one of two processes: High Pressure High Temperature (HPHT), which replicates the crushing conditions inside the Earth’s mantle, or Chemical Vapor Deposition (CVD), which builds a diamond carbon atom by carbon atom inside a vacuum chamber. Both methods start with a tiny diamond seed and end with a chemically identical stone that passes the same gemological tests as a mined diamond. The difference is physics: HPHT squeezes, CVD deposits. Understanding each stage matters when you are comparing stones, reading a grading report, or simply curious about where a lab diamond actually comes from.

What happens during the HPHT process, step by step?

HPHT is the older of the two methods, first used successfully for industrial diamonds in 1955. The goal is to copy what happens roughly 100 miles below the Earth’s surface, where carbon crystallizes under enormous geological force, but to do it in days rather than billions of years.

Step 1: Seed placement and chamber loading

A small diamond seed, often a fragment from a previous growth run, is placed inside a growth capsule along with a high-purity carbon source (typically graphite) and a metal catalyst. The catalyst, usually a mixture of iron, nickel, or cobalt, is essential: it lowers the temperature at which carbon dissolves and then re-deposits onto the seed in diamond form.

Step 2: Applying pressure and heat

The capsule goes into one of three press types: a belt press, a cubic press, or a split-sphere (BARS) press. The chamber is then brought to roughly 5 to 6 gigapascals of pressure, equivalent to more than 870,000 pounds per square inch, while temperature climbs to between 1,300°C and 1,600°C. At those conditions, the graphite dissolves into the molten metal catalyst, and carbon atoms begin to precipitate onto the seed crystal, building outward layer by layer.

Step 3: Growth and extraction

The diamond grows over a period of days to a few weeks, depending on the target carat size. When the press opens, what comes out is a rough crystal with a combination of cubic and octahedral faces, coated in residual graphite and metal. That rough stone goes on to cutting and polishing, the same as any mined rough.

What happens during the CVD process, step by step?

CVD arrived commercially in the 1980s and now accounts for the majority of gem-quality lab grown diamonds on the market. It works at much lower pressures than HPHT, which reduces equipment costs, though it introduces its own complications at the color stage.

Step 1: Seed preparation

A thin, wafer-like slice of diamond, often an HPHT-grown seed plate, is cleaned and mounted inside a vacuum chamber. Even microscopic dust particles can disrupt the crystal lattice, so the chamber is evacuated to near-vacuum before anything else happens.

Step 2: Gas introduction and plasma activation

The chamber fills with a mixture of hydrogen and methane gas, typically at around 900°C to 1,200°C. A microwave beam then ionizes the gas, creating a plasma cloud. Inside that plasma, the hydrocarbon molecules break apart, releasing free carbon atoms.

Step 3: Layer-by-layer carbon deposition

The freed carbon atoms migrate toward the cooler diamond seed and attach to its surface. The diamond grows in thin layers, one atomic sheet at a time. This is slower and more controlled than HPHT growth, and it gives technicians tighter command over purity and structure. A gem-quality CVD diamond typically takes three to four weeks to reach a cuttable size.

Step 4: Post-growth color treatment (where applicable)

CVD growth tends to produce diamonds with a brownish undertone caused by nitrogen-vacancy defects and structural dislocations that form during deposition. Many CVD diamonds then go through an HPHT annealing step: the already-formed stone is placed back into a high-pressure, high-temperature environment, which reorganizes the defects at the atomic level and permanently improves the color. Nothing is added to the diamond; the lattice is simply corrected. A full IGI grading report will note whether post-growth treatment was applied, so buyers can see exactly what they are purchasing.

How does cutting, polishing, and certification work?

Once a rough lab grown diamond comes out of either process, it goes through the same skilled finishing stages as a mined stone.

Laser sawing and bruting shape the rough into the chosen form, whether round brilliant, oval, cushion, or any other cut. Hand polishing then opens up the facets that control how light enters and exits the stone. A poorly cut diamond wastes the optical potential of even chemically perfect carbon, which is why cut is graded independently of the other three Cs.

After polishing, the stone goes to a gemological laboratory for certification. For lab grown diamonds, IGI (International Gemological Institute) is the dominant certification body, used for the majority of the market. IGI’s grading process evaluates the four Cs: carat weight, color on the D-to-Z scale, clarity at 10x magnification, and cut. Multiple gemologists assess each stone independently, with a grade confirmed only when there is sufficient agreement among them. The final report also explicitly states the growth method, CVD or HPHT, and discloses any post-growth treatment. A laser inscription on the diamond’s girdle ties the physical stone to its report number.

All lab grown diamonds at Ouros Jewels carry IGI certification, so every grading detail is documented before a stone is set into jewelry.

Are CVD and HPHT diamonds physically identical to mined diamonds?

Yes, in every chemically and physically meaningful way. Both processes produce carbon atoms arranged in the same cubic crystal lattice that gives all diamonds their hardness of 10 on the Mohs scale and their characteristic optical behavior. A gemologist cannot distinguish a lab grown diamond from a mined one using standard visual inspection or a basic diamond tester. Only specialized spectroscopic equipment, the kind used by IGI and GIA, can identify the growth method by detecting trace inclusions or fluorescence patterns specific to each process.

The distinction that matters to buyers is between the two lab methods themselves. HPHT diamonds tend to emerge with fewer color issues and generally require no post-growth treatment, though the machinery is expensive and growth is slower. CVD diamonds are faster to produce and offer excellent purity control, but most colorless CVD stones on the market have gone through the annealing step described above. Neither is inferior as a finished gem, provided the final grading report reflects the quality you are paying for.

The Seed Is Everything

Every lab grown diamond, regardless of which process made it, traces back to a single diamond seed. That seed determines the crystal orientation, influences the growth rate, and shapes the quality ceiling of the finished stone. The sophistication of the equipment matters, the precision of the gas ratios or the pressure curves matters, but the seed is where the diamond’s identity begins. For buyers, the practical takeaway is straightforward: read the IGI report, note the growth method, check for post-growth treatment disclosure, and let the four Cs tell the rest of the story. If you want a starting point for a customized lab grown diamond ring, the science behind the stone is already locked in before the jeweler ever touches it.

Frequently Asked Questions

How long does it take to grow a lab grown diamond?

HPHT diamonds typically grow in a few days to a few weeks, depending on the target carat size. CVD diamonds generally take three to four weeks to reach a cuttable size, and some may need additional time for the post-growth HPHT annealing step that corrects color before the stone goes to a cutter.

Can you tell a CVD diamond from an HPHT diamond just by looking at it?

Not reliably with the naked eye. Gemological laboratories use photoluminescence spectroscopy and DiamondView fluorescence imaging to distinguish the two growth types. CVD diamonds may show distinctive strain patterns and needle-like inclusions under magnification, while HPHT diamonds can carry traces of the metal catalyst used during growth, though these are usually submicroscopic.

Does post-growth HPHT treatment make a CVD diamond lower quality?

Not necessarily. The annealing process corrects structural defects at the atomic level without adding or removing material from the diamond. The result is a permanently improved color grade. What matters is that the treatment is disclosed on the IGI certificate, which it should be, so you can evaluate the stone’s quality accurately.

What is a diamond seed and where does it come from?

A diamond seed is a small fragment or thin slice of an existing diamond, often produced in a previous lab growth cycle. It provides the crystal template that guides how new carbon atoms arrange themselves during growth. Without a seed, carbon atoms would not organize into the ordered lattice structure that makes a diamond hard and optically brilliant.

Why do lab grown diamonds cost less than mined diamonds if the process is so technical?

The cost difference comes from the supply chain, not the science. Mining requires extraction, heavy machinery, land rights, and long logistics chains. Lab growth happens in a controlled facility where production can scale, waste is minimal, and the time from seed to polished stone is measured in weeks rather than the years a mined diamond spends moving through the supply chain. The diamond itself is physically identical; the overhead is not.