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

Lab grown diamonds are made by one of two processes: High Pressure High Temperature (HPHT), which mimics the conditions deep in the Earth’s mantle using pressures around 5 to 6 gigapascals and temperatures of 1,300 to 1,600°C, or Chemical Vapor Deposition (CVD), which grows a diamond atom by atom inside a low-pressure vacuum chamber filled with carbon-rich gas. Both methods start with a diamond seed crystal and produce a stone that is chemically, physically, and optically identical to a mined diamond.

The two processes differ in equipment, timeline, and the type of inclusions they tend to produce, but the end result passes the same gemological tests. Understanding both gives buyers a clearer picture of what they are purchasing and why IGI certification matters as the final quality checkpoint.

What is the HPHT process for growing a diamond, step by step?

HPHT was the first laboratory method ever used to synthesize diamond, developed in the 1950s, and it works by recreating the pressure and heat conditions found roughly 100 miles below the Earth’s surface.

Step 1: Seed selection. A small, high-quality diamond crystal is chosen as the seed. It sits at the cooler end of a growth capsule alongside a carbon source, usually high-purity graphite, and a metal solvent-catalyst, typically an alloy of iron, nickel, and cobalt.

Step 2: Applying pressure and heat. The capsule goes inside a multi-ton press. The press raises pressure to roughly 5 to 6 gigapascals (about 50,000 to 60,000 times standard atmospheric pressure) while heating the cell to between 1,300 and 1,600°C. Three press designs are used in commercial production: the belt press, the cubic press, and the BARS apparatus, each capable of sustaining those extreme conditions.

Step 3: Carbon dissolution. At those conditions, the graphite dissolves into the molten metal catalyst. The catalyst does something important here: it lowers the threshold at which carbon transitions from graphite to diamond, making the reaction feasible in a laboratory rather than requiring literal mantle depths.

Step 4: Crystal growth. Because the seed sits at a slightly cooler temperature than the carbon source, a thermal gradient forms across the cell. Carbon migrates through the molten metal toward the cooler seed and crystallizes onto it, building the diamond layer by layer. This stage typically runs for several days to a few weeks depending on the target carat weight.

Step 5: Cooling and extraction. Once the target size is reached, the press is slowly cooled under pressure before the pressure is released. The rough diamond crystal is then extracted from the capsule, still surrounded by solidified metal flux that has to be removed mechanically and chemically.

What is the CVD process for growing a diamond, step by step?

CVD takes a different route entirely. Instead of brute-force pressure, it uses plasma chemistry at pressures well below atmospheric. Today, CVD is the method behind most gem-quality lab diamonds sold in the market.

Step 1: Seed preparation. A thin slice of diamond, typically 3 to 5 millimeters across and often cut from a previous lab-grown stone, is cleaned thoroughly. Any surface defect at this stage tends to propagate into the finished crystal as an inclusion, so cleanliness is not optional.

Step 2: Chamber setup. The seed is placed inside a sealed vacuum reactor. The chamber is filled with a mixture of methane (CH₄) and hydrogen (H₂). Methane supplies the carbon atoms; hydrogen plays a stabilizing role, preferentially etching away non-diamond carbon (graphite) that would otherwise contaminate the growing crystal.

Step 3: Plasma generation. Microwaves energize the gas mixture until it forms a plasma, a superheated cloud of reactive gas that reaches around 3,000°C. The plasma breaks the methane molecules apart, freeing individual carbon atoms and fragments.

Step 4: Carbon deposition. Freed carbon atoms migrate downward and settle onto the diamond seed, locking into the crystal lattice one atomic layer at a time. The growth rate runs at roughly 0.1 to 0.3 millimeters per day. A 1-carat round brilliant typically takes two to four weeks to grow this way.

Step 5: Interruption and surface cleaning. Technicians stop the process periodically to remove any graphite that has crystallized on the surface. Each stop-start cycle keeps the growing diamond clean. The full growth run for a single batch can involve several of these cycles.

Step 6: Rough crystal extraction. When the crystal reaches the target thickness, the reactor is powered down. The rough stone looks nothing like a finished diamond at this point: it is a dark, cube-like slab that needs significant work before it resembles jewelry.

How are lab grown diamonds cut, polished, and certified after growth?

The rough stone from either process goes through the same downstream steps as a mined diamond.

Cutting and polishing

A diamond cutter maps the rough crystal using imaging software to plan where to cut for maximum yield and optical performance. The stone is then cleaved or sawn, bruted (shaped on a lathe against another diamond), and faceted using a spinning wheel coated with diamond powder. Polishing follows the same sequence. A 1-carat round brilliant ends up with 57 or 58 facets, each angle calculated to maximize light return.

IGI certification

Once polished, the stone goes to a gemological laboratory. IGI (the International Gemological Institute) has been certifying lab-grown diamonds since 2005 and is the dominant certifier in this category. Graders evaluate the stone under standardized conditions against the 4Cs: carat weight, color (on the D-to-Z scale), clarity (FL to I3), and cut grade. IGI also uses advanced spectroscopic testing to confirm the stone’s lab-grown origin and to identify whether it is CVD or HPHT grown.

Since October 2025, GIA replaced full 4Cs grading for lab-grown diamonds with a simplified two-tier system (“Premium” or “Standard”). IGI continues to issue full 4Cs reports using the same scales applied to natural diamonds, which makes IGI grading more practically useful for anyone comparing specific stones. Most IGI-certified diamonds also receive a laser inscription on the girdle, a microscopic number invisible to the naked eye that ties the physical stone to its grading report.

All lab-grown diamond jewelry from Ouros Jewels carries IGI certification, so buyers receive that full 4Cs report with every purchase.

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

Neither process is categorically superior. The differences are real but narrower than most buyers expect.

HPHT diamonds tend to produce more consistent color without post-growth treatment, and the process has a longer commercial track record. CVD diamonds, grown at lower pressures, sometimes show a slight brown or gray tint in the rough that requires a brief HPHT color treatment after growth to reach D-to-F color grades. IGI reports now explicitly note whether a stone is “as grown” or has undergone post-growth treatment, so that information is not hidden.

In terms of hardness, refractive index, thermal conductivity, and chemical composition, CVD and HPHT diamonds are identical to each other and to mined diamonds. A gemologist cannot tell them apart from a mined stone with the naked eye; it takes spectroscopic equipment to identify the growth method.

The Process Creates a Real Diamond, Not a Simulant

Both HPHT and CVD produce diamonds made entirely of carbon atoms arranged in a cubic crystal lattice, the same structure found in every mined diamond. That is the fact that separates a lab-grown diamond from a simulant like cubic zirconia or moissanite, which have different chemical compositions entirely.

The growth method is a manufacturing detail. The diamond itself, once cut and certified, carries the same optical and physical properties that have made diamonds the standard for fine jewelry for centuries. What changes is the supply chain: no mining, no environmental disruption at the extraction site, and a price that reflects weeks of controlled laboratory production rather than decades of geological luck.

For shoppers who want to see what that process produces in finished form, the IGI Certified Round Lab Grown Diamond Stud Earrings at Ouros Jewels offer a concrete starting point, each stone grown, cut, and graded through exactly the process described above.

Frequently Asked Questions

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

HPHT diamonds typically grow in several days to a few weeks depending on the target carat weight. CVD diamonds generally require two to four weeks for the growth phase alone, with a 1-carat stone averaging about three weeks. Cutting, polishing, and IGI certification add additional time before the stone reaches a jeweler.

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

No. CVD and HPHT diamonds are visually indistinguishable from each other and from mined diamonds. Gemological laboratories use spectroscopic equipment, not visual inspection, to identify the growth method. The IGI grading report will state which process was used, but the stone itself shows no visible difference.

Does the diamond seed used in CVD or HPHT become part of the finished diamond?

Yes, the seed crystal remains at the base of the finished stone. In CVD, the seed is a thin diamond slice roughly 3 to 5 millimeters across; in HPHT, it is a small diamond crystal placed at the cooler end of the growth capsule. Both seeds are incorporated into the larger crystal and are indistinguishable from the surrounding material once growth is complete.

Why do some CVD lab-grown diamonds need post-growth HPHT treatment?

CVD growth sometimes produces a brown or gray tint caused by lattice strain in the crystal. A brief HPHT treatment at high temperature relieves that strain and shifts the color toward the colorless D-to-F range. IGI grading reports since 2021 explicitly disclose whether a stone is “as grown” or has undergone this treatment, so buyers have full information before purchasing.

Are lab-grown diamonds graded on the same scale as mined diamonds?

Yes. IGI grades lab-grown diamonds using the same D-to-Z color scale and FL-to-I3 clarity scale used for natural diamonds. Cut grades follow the same Excellent-to-Poor system. The only addition on an IGI lab-grown report is the disclosure of laboratory-grown origin and the growth method, giving buyers all the information they would get for a mined stone plus that extra transparency.