Lab-Grown Diamonds · The Science

How Are Lab-Grown Diamonds Made? CVD and HPHT Explained (2026)

By Tushar Leclerc · 11 min read

Last Updated: September 12, 2026

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A mined diamond forms over one to three billion years, roughly 150 kilometres down in the earth's mantle, under crushing pressure and heat. A lab-grown diamond reaches the identical result, the same pure-carbon crystal, in a few weeks. This is how that compression of a billion years into a fortnight actually works, step by step, without the marketing gloss.

The important thing to hold onto throughout: neither method is an imitation. Both produce genuine diamond, the same crystal structure, the same hardness of 10, the same optical properties. The methods differ in how they assemble the carbon, not in what they produce.

It all starts with a seed

Both methods begin the same way, with a diamond seed: a thin, flat sliver of an existing diamond, often itself lab-grown. Everything that follows is the seed growing larger as new carbon attaches to it in the correct crystal arrangement.

This is the key idea behind the whole process. You are not manufacturing something diamond-like from scratch. You are giving carbon atoms the exact conditions they need to keep building the diamond lattice that the seed already started, the same way a sugar crystal grows in a jar of syrup, only the "syrup" here is either molten carbon or a glowing plasma of gas.

How lab-grown diamonds are made: the two methods

Two techniques dominate gem-quality production. HPHT is the original, developed for industry in the 1950s; CVD is newer, reaching gem quality in the mid-1990s and now the more common method for jewellery.

Neither is "better" for quality. Both are used commercially, both are certified by IGI and GIA, and the growth method is simply recorded on the stone's report. What differs is the machinery and the path the carbon takes to the crystal.

HPHT, step by step

HPHT is the brute-force method: it recreates the extreme conditions that form diamonds naturally, then does in weeks what the earth does in eons.

• 1. The seed is set in carbon. A diamond seed is placed in a growth cell along with a solid carbon source, usually high-purity graphite, and a metal catalyst, typically iron, nickel or cobalt.
• 2. The press applies enormous pressure. The cell goes into a specialised press, a cubic, belt or split-sphere (BARS) design, which drives the pressure to around 5 to 6 GPa, roughly 1.5 million pounds per square inch. That is the pressure found about 150 kilometres underground.
• 3. Heat melts the carbon. The chamber is taken to 1,300 to 1,600°C. The metal catalyst melts and dissolves the graphite, freeing its carbon atoms.
• 4. Carbon crystallises onto the seed. The dissolved carbon migrates through the molten metal toward the cooler seed and precipitates onto it, atom by atom, extending the diamond lattice. Over days to a couple of weeks, a rough crystal grows outward from the seed.

HPHT is prized for producing high colour grades, and the same process is also used separately to improve the colour of stones after growth.

Round SolitaireFrom $1,288 Oval HaloFrom $1,530 Cushion SolitaireFrom $1,369 Emerald SolitaireFrom $1,369 Oval Three-StoneFrom $1,450

CVD, step by step

CVD takes the opposite approach. Instead of crushing carbon into shape, it coaxes carbon out of a gas and lets it settle onto the seed one layer at a time, at far lower pressure.

• 1. Seeds go into a vacuum chamber. Thin diamond seed plates are placed in a sealed chamber, which is pumped down to near vacuum.
• 2. Carbon-rich gas fills the chamber. The chamber is filled with a hydrocarbon gas mixture, mostly methane and hydrogen, and heated to roughly 800 to 1,200°C.
• 3. Energy creates a plasma. Microwave energy ionises the gas into a glowing plasma. This breaks the methane apart and frees its carbon atoms.
• 4. Carbon rains down onto the seed. The freed carbon settles onto the seed plate and bonds into the diamond lattice, building the crystal upward layer by layer. Over several weeks the diamond grows in flat, tabular form.

CVD runs at much lower pressure than HPHT and gives precise control over the growing conditions, which is why it is favoured for high-purity, high-clarity stones. Some CVD diamonds receive a post-growth HPHT treatment to refine their colour, which the certificate will note.

How long it takes

The headline figure is the striking one: what takes the earth one to three billion years takes a lab a matter of weeks. A rough crystal of gem size typically grows in two to four weeks, depending on the method, the size targeted, and the quality being aimed for. Larger and higher-clarity stones take longer, because the conditions have to be held stable for the whole growth run and any instability can flaw the crystal.

That is growth time only. Cutting and polishing the rough into a finished gem adds more time on top, and it is skilled hand-work no faster for a lab stone than a mined one.

From rough crystal to finished gem

Here is the part most people miss: once a lab diamond finishes growing, the rest of its journey is identical to a mined diamond's. The reactor produces a rough crystal, not a sparkling gem. That rough is then:

• Planned and mapped, often with lasers, to decide how to cut it for the best yield and brilliance.
• Cut and faceted by the same cutters, using the same equipment, applying the same proportions that govern a mined stone's sparkle.
• Polished to its final finish.
• Graded and certified by an independent lab such as IGI or GIA, on the same 4Cs of cut, colour, clarity and carat.

Cut is where a diamond's sparkle is made, lab or mined alike, which is why it is the C worth prioritising when you buy. Our guide to the 4Cs covers how to read a grading report and where to spend.

How to know which method made your diamond

You cannot tell by looking, and neither can a jeweller with a loupe. A well-grown CVD and a well-grown HPHT diamond of the same grades are visually identical, to each other and to a mined stone. The difference is only detectable with specialist laboratory equipment that reads the tiny trace features left by each growth process.

Which is exactly why the certificate matters. Every IGI report for a lab-grown diamond states the growth method, records the full 4Cs, notes any post-growth treatment, and carries a laser-inscribed number on the girdle that ties the paper to that one stone. Every Riona diamond is IGI-certified and arrives with that report. For the wider picture of what a lab diamond is and how it compares to mined, see our complete guide to lab-grown diamonds.

Round SolitaireFrom $1,288 Hidden Halo PavéFrom $1,611 Oval Three-StoneFrom $1,450 Emerald SolitaireFrom $1,369 Oval HaloFrom $1,530

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