How Are Lab Grown Diamonds Made Step by Step: CVD and HPHT Processes Fully Explained

Lab grown diamonds are made using one of two processes: High Pressure High Temperature (HPHT), which replicates the geological conditions of Earth’s mantle, or Chemical Vapor Deposition (CVD), which grows carbon atom by atom from a gas plasma inside a sealed reactor. Both start with a diamond seed, apply precise heat and carbon, and produce stones that are chemically, physically, and optically identical to mined diamonds. The difference is in the mechanics, the timelines, and the resulting crystal structure.

Neither method is new. HPHT dates to the 1950s, while CVD became commercially viable decades later and now accounts for the majority of gem-quality lab diamonds sold in 2026. Understanding both matters if you want to know what you are actually buying.

What happens during HPHT diamond growth?

HPHT mimics the way diamonds form naturally, roughly 100 miles below Earth’s surface, by applying extreme pressure and heat to a carbon source inside a sealed growth capsule. The process starts with a small diamond seed, usually a flat sliver of an existing diamond, placed inside a metal capsule alongside a carbon-rich material (most commonly high-purity graphite) and a metal catalyst flux of iron, cobalt, or nickel.

The capsule is loaded into a multi-anvil press and subjected to temperatures between 1,300°C and 1,600°C and pressures of 5 to 6 GPa, which is roughly equivalent to 70,000 times normal atmospheric pressure. Under those conditions, the metal catalyst melts, the graphite dissolves into it, and carbon atoms migrate through the molten metal toward the cooler diamond seed, where they crystallize layer by layer into a new diamond crystal.

The growth typically takes anywhere from a few days to several weeks, depending on the target carat size. Once the crystal reaches the desired dimensions, the press is cooled slowly, the capsule is opened, and the rough diamond is extracted. Trace amounts of the metal catalyst can remain as microscopic inclusions, though they are usually invisible to the naked eye. The rough stone then goes to a cutting and polishing facility before moving to certification.

What happens during CVD diamond growth?

CVD takes a different route entirely. Instead of pressure, it uses a near-vacuum chamber and a carbon-containing gas to build a diamond one atomic layer at a time.

A diamond seed, typically a thin plate 3 to 5 millimeters across, is placed inside the reactor. The chamber is filled with a gas mixture of roughly 99% hydrogen and around 1% methane. Microwaves heat that mixture until it reaches plasma state, breaking the methane molecules apart and freeing individual carbon atoms. Those atoms settle onto the seed crystal and bond to it in the same tetrahedral arrangement that makes diamond the hardest natural material on Earth.

CVD grows at much lower pressure than HPHT and at moderate temperatures, generally in the 700°C to 900°C range inside the plasma zone. A 1-carat stone takes roughly two to four weeks to grow at a rate of about 0.1 millimeters per hour. The result is a square-ish rough crystal that looks nothing like a finished gem. Some CVD diamonds come out with a slight brownish tint from structural strain during growth; those stones often receive a short HPHT post-growth treatment to correct the color before cutting.

Because CVD operates at low pressure and builds the crystal gradually, it tends to produce fewer metallic inclusions than HPHT and can achieve very high clarity grades consistently.

How does a rough lab diamond become a finished gem?

The cutting and polishing stage is where the raw crystal becomes recognizable as jewelry. It follows the same sequence used for mined diamonds, and the skill involved is identical.

The process moves through bruting (spinning two diamonds against each other to shape the girdle), blocking (placing the first major facets), and then brillianteering (adding the smaller facets that create the finished optical pattern). A round brilliant, for example, ends up with 57 or 58 facets cut to precise angles that determine how light reflects and refracts inside the stone. A poorly cut diamond of excellent color and clarity will still look flat; the cut grade is what releases the optical performance.

After polishing, the stone is cleaned and sent to an independent grading laboratory.

What does IGI certification cover, and why does it matter?

Certification is where the production process becomes a documented, verifiable record. For lab grown diamonds, the International Gemological Institute (IGI) is the dominant certifying body globally. IGI pioneered lab grown diamond grading in 2005 and evaluates each stone on the standard 4Cs: carat weight, color (graded on the D-to-Z scale), clarity (assessed at 10x magnification), and cut.

Beyond the 4Cs, the IGI report identifies the growth method, either HPHT or CVD, and notes any post-growth treatments. The lab uses spectroscopic testing and analysis of optical characteristics to distinguish lab grown stones from natural ones and to confirm the production method. Every certified stone receives a laser inscription on its girdle: the IGI report number plus the words “LAB GROWN,” readable under a 10x loupe and verifiable in seconds at igi.org.

That inscription is what makes the certification traceable rather than just decorative. Every lab grown diamond at Ouros Jewels carries IGI certification, so the full record of growth method, treatment history, and final grades travels with each stone before it is set into jewelry.

CVD vs. HPHT: Which Process Produces a Better Diamond?

Neither is categorically better. The two methods produce diamonds with the same chemical composition and the same hardness of 10 on the Mohs scale. The practical differences come down to inclusions, color tendencies, and cost.

HPHT tends to produce more consistent color straight out of the press, but the metal catalyst can leave microscopic metallic inclusions. CVD produces fewer inclusions on average and is easier to scale, but some stones require post-growth color treatment. A well-cut, well-graded diamond from either process will perform identically in a ring or earring setting. The IGI report tells you exactly which method was used, so you are never guessing.

Lab Grown Diamonds Are Real Diamonds, Made Differently

The process that creates a lab grown diamond, whether HPHT or CVD, produces the same carbon crystal structure as a diamond formed over billions of years underground. The seed, the carbon source, and the energy input change; the outcome does not. What you get is a stone with the same optical properties, the same hardness, and the same chemical makeup, graded by the same standards and inscribed with a traceable report number.

For shoppers who want to see that documentation applied to a finished piece, the IGI Certified Round Lab Grown Diamond Stud Earrings are a good starting point: each stone is CVD or HPHT grown, fully certified, and the report travels with it.

Frequently Asked Questions

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

HPHT diamonds typically take a few days to several weeks to grow, depending on the target size. CVD diamonds for gem use take roughly one to four weeks in the reactor. Add cutting, polishing, and IGI certification time, and the full process from seed to certified, finished stone is usually six to ten weeks.

Can you tell the difference between a CVD diamond and an HPHT diamond just by looking at them?

Not with the naked eye, and not reliably even under a standard loupe. Both produce optically identical diamonds. Gemological labs like IGI use spectroscopic testing and analysis of growth patterns to identify the method. The growth process is noted on the IGI certificate, so any certified stone will tell you exactly how it was made.

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

No. Post-growth HPHT treatment is a color-correction step used on some CVD diamonds that develop a brownish tint during growth. The treatment does not alter the diamond’s hardness, chemical structure, or long-term durability. IGI notes any post-growth treatment on the certificate, so the process is fully disclosed and does not affect the stone’s graded quality.

Are lab grown diamonds graded the same way as mined diamonds?

Yes. IGI and other grading labs apply the same 4Cs criteria, carat weight, color, clarity, and cut, to lab grown diamonds as they do to mined stones. The only additions on a lab grown report are the identification of the stone as laboratory-created, the growth method (CVD or HPHT), and any post-growth treatments.

Why do lab grown diamonds cost less than mined diamonds if they are chemically identical?

The cost difference comes from the supply chain, not the material. A CVD reactor can produce a steady supply of diamonds in weeks, compared to the mining, transport, and trading costs embedded in a mined stone. Fewer intermediaries and a faster, more predictable production cycle reduce the price without changing what the diamond actually is.