CVD vs HPHT — The Two Ways Science Grows a Lab Grown Diamond
Introduction: Same Diamond, Two Very Different Journeys
If you've started researching lab grown diamonds — and welcome, because you are making an excellent life decision — you've likely stumbled across two mysterious acronyms: CVD and HPHT.
They sound like something from a chemistry exam you didn't study for. Or perhaps two new airport codes for destinations you've never heard of. (CVD — surely somewhere warm? HPHT — industrial Scandinavia, possibly?)
In reality, CVD and HPHT are the two scientific methods by which lab grown diamonds are created. They are both legitimate, both produce genuine diamonds, and both result in stones that are chemically, physically, and optically identical to diamonds formed in the earth. But the paths they take to get there are fascinatingly different — one mimics the violent heat and crushing pressure of the Earth's mantle, the other grows diamonds from a cloud of superheated gas, atom by atom, like the world's most glamorous 3D printer.
In this post — the second in H&H Jewellery's definitive 15-part Lab Grown Diamonds series — we're pulling back the curtain on both processes. By the end, you'll understand what's happening inside those laboratory chambers, why it matters, and how to make sense of it all when you're shopping for your perfect stone.
Let's get into the science. It's better than it sounds, we promise.
First, a Quick Recap: What Makes a Diamond a Diamond?
Before we dive into the how, a fast reminder of the what.
A diamond is pure carbon, organised into a rigid, three-dimensional crystal lattice. This structure — forged under extreme conditions — is what gives diamonds their legendary hardness (a perfect 10 on the Mohs scale), their extraordinary light performance, and their breathtaking durability.
In nature, this process happens approximately 140 to 240 kilometres below the Earth's surface, where temperatures sit around 1,000–1,400°C and pressures reach 5.5 to 8.0 GPa (gigapascals). For context: 1 GPa is roughly 145,000 pounds per square inch. Natural diamonds form over timescales of 1 to 3 billion years before volcanic activity brings them close enough to the surface to be mined.
Science's challenge was simple to state and extraordinarily difficult to solve: replicate those conditions — or find an entirely different route to the same destination.
Two solutions emerged. One followed nature's playbook. The other rewrote it entirely.
Method One: HPHT — Brute Force Alchemy
What Does HPHT Stand For?
HPHT stands for High Pressure, High Temperature. The name is admirably literal. It is essentially the laboratory equivalent of what the Earth does over a billion years — compressed, controlled, and completed in a matter of weeks.
How Does the HPHT Process Work?
The HPHT process begins with a diamond seed — a tiny sliver of existing diamond crystal. This seed is placed into a specially designed growth chamber along with a carbon source (typically high-purity graphite) and a metal catalyst flux — usually a mixture of metals such as iron, nickel, or cobalt.
The chamber is then subjected to conditions that are, by any reasonable measure, extreme:
- Temperature: 1,300–1,600°C (approximately the temperature of Earth's upper mantle)
- Pressure: 5–6 GPa — equivalent to 50,000 to 60,000 times normal atmospheric pressure
To put that pressure figure in terms that mean something: the GIA describes it as roughly equivalent to the pressure exerted by a commercial jet aircraft balanced on the tip of a single finger. An entire aircraft. One fingertip.
Under these conditions, the metal catalyst melts into a liquid flux that dissolves the carbon from the graphite source. The dissolved carbon then migrates through the molten metal toward the cooler diamond seed, where it precipitates out and begins growing — layer by layer — into a new diamond crystal.
Depending on the desired size, this process typically takes anywhere from a few days to a few weeks.
What Does an HPHT Diamond Look Like When It's Growing?
Here's where it gets visually interesting. Because HPHT diamonds grow simultaneously in multiple directions — specifically, 14 different growth directions — they form in a distinctive cuboctahedral shape: a geometric solid with a combination of cubic and octahedral faces. Think of a diamond-shaped crystal in the most literal sense — angular, multifaceted, raw.
The metal catalyst that facilitates growth can leave behind metallic inclusions — tiny traces of iron, nickel, or cobalt captured within the crystal. These inclusions appear dark in transmitted light but exhibit a metallic sparkle, and they are occasionally magnetic, which is one way gemologists can identify an HPHT-grown stone under laboratory conditions.
Fun Fact 💎
An HPHT crystal weighing 115.81 carats was grown in Ukraine — at the time, the largest HPHT-grown diamond crystal in laboratory history. For reference, the famous Koh-i-Noor diamond (now part of the British Crown Jewels) weighs 105.6 carats. Science, on this occasion, outdid the Earth.
HPHT for Colour Treatment: A Bonus Superpower
Here's something that surprises many people: the HPHT process isn't only used to grow diamonds from scratch. It's also used as a post-growth treatment to improve the colour of existing diamonds — both mined and lab grown.
Many CVD-grown diamonds (more on those shortly) emerge from the reactor with a slightly brownish or greyish tint caused by structural distortions that occur during growth. The HPHT treatment process applies intense heat and pressure to realign the crystal structure, eliminating or reducing that tint and producing a whiter, more colourless stone. It's a stable, permanent treatment — the improved colour does not revert over time.
When you see an IGI certificate for a CVD diamond that states "post-growth treatment," this is what it refers to. It's standard practice, widely accepted, and does not negatively affect the quality or value of the stone.
Method Two: CVD — The Art of Growing Diamonds From Gas
What Does CVD Stand For?
CVD stands for Chemical Vapour Deposition. If HPHT is diamond-making by brute force, CVD is diamond-making by extraordinary precision. Rather than crushing and heating carbon into submission, CVD grows diamonds atom by atom from a cloud of superheated gas — a process that has no equivalent anywhere in nature.
This is not how diamonds form on Earth. This is purely a human invention. And it is, frankly, one of the more remarkable things our species has figured out how to do.
How Does the CVD Process Work?
The CVD process also begins with a diamond seed — a thin slice of diamond crystal placed inside a vacuum-sealed reactor chamber. The chamber is then filled with a mixture of carbon-containing gases, primarily methane (CH₄) and hydrogen (H₂), at controlled low pressures.
Here is where the magic happens.
A microwave beam (operating at 2.45 GHz — the same frequency as a domestic microwave oven, incidentally) bombards the gas mixture, generating enormous energy. The gas transforms into a superheated plasma cloud — reaching temperatures of around 900–1,200°C — and the plasma begins breaking down the methane molecules, releasing individual carbon atoms.
Those liberated carbon atoms rain down onto the diamond seed below the plasma cloud, attaching to the crystal lattice in precisely the right arrangement. Layer by layer — one atomic layer at a time — the diamond grows upward from the seed, like a crystalline skyscraper being assembled one floor at a time.
This layer-by-layer growth gives CVD diamonds a distinctive cubic structure with a single upward growth direction, quite different from the multi-directional growth pattern of HPHT crystals.
The Microwave Plasma Detail
The use of a microwave plasma reactor in CVD diamond growth is one of those technical details that sounds dry until you sit with it for a moment.
Consider: you are using microwave radiation — the same electromagnetic energy that reheats last night's curry — to superheat a carbon-bearing gas into plasma, from which individual carbon atoms precipitate out and self-organise, through atomic-scale bonding forces, into one of the hardest and most optically perfect materials in existence.
The hydrogen gas in the CVD mix plays a critical supporting role. As confirmed in peer-reviewed materials science research, hydrogen atoms stabilise the diamond growth and actively suppress the formation of graphite — the other form of pure carbon that would otherwise compete to crystallise. Without the hydrogen, you'd end up with a graphite electrode, not a diamond.
Fun Fact 🔬: A group of scientists once grew a CVD diamond using tequila as the carbon source. The diamond wasn't gem quality — the impurities in the spirit made sure of that — but the experiment confirmed that any hydrocarbon will do in a pinch. Science: occasionally delightful.
The Growth Chamber and Cleaning Cycles
CVD diamond growth is not a set-and-forget process. As the diamond grows, a layer of graphite periodically builds up around the crystal — if left unchecked, this would contaminate the stone and cause it to grow as a polycrystalline mass rather than a single clean crystal.
The GIA notes that CVD crystals must therefore be removed from the chamber multiple times during growth so this graphite layer can be cleaned off before growth resumes. It's a more labour-intensive process than HPHT — but the level of control it offers is unmatched.
The result is typically a diamond with exceptional purity and very few inclusions, classified as a Type IIa diamond — the purest category of diamond that exists. In nature, Type IIa diamonds are extremely rare; in the CVD lab, they're the norm.
HPHT vs CVD: The Side-By-Side Breakdown
Let's cut through to the practical comparison.
Temperature & Pressure
- HPHT: 1,300–1,600°C, 5–6 GPa (50,000–60,000 atmospheres)
- CVD: 900–1,200°C, 0.1–0.3 GPa (much lower pressure)
HPHT operates at dramatically higher pressures. CVD requires less brute force, relying instead on plasma chemistry.
Growth Direction & Crystal Shape
- HPHT: 14 growth directions, cuboctahedral crystal shape
- CVD: Single upward growth direction, cubic crystal shape
Diamond Type & Purity
- HPHT: Typically produces Type IIa or Type IIb diamonds (very pure, though may contain boron)
- CVD: Consistently produces Type IIa diamonds — chemically the purest diamond classification
Inclusions
- HPHT: May contain metallic inclusions (iron, nickel, cobalt) — dark in appearance, occasionally magnetic
- CVD: May contain dark graphite inclusions; no metallic inclusions
Colour
- HPHT: Can produce colourless diamonds directly; nitrogen exposure during growth can cause a yellowish tint in some stones
- CVD: Often requires post-growth HPHT treatment to achieve optimum colourlessness; without treatment, may have a brownish or greyish tinge
Post-Growth Treatment
- HPHT: Can be grown "as-is" with no treatment required (noted on IGI certificate as "As Grown")
- CVD: Approximately 75% of CVD diamonds undergo HPHT treatment post-growth to improve colour
Cost of Production
- HPHT: More expensive to produce due to high-pressure equipment requirements
- CVD: Generally more cost-effective to produce; lower equipment costs and operating pressures
Time to Grow
- HPHT: A few days to a few weeks
- CVD: Several weeks, due to layer-by-layer growth and cleaning cycles
So Which Is Better — CVD or HPHT?
Here's the honest answer: neither is categorically better than the other. Both methods produce genuine diamonds. Both can produce stones of exceptional quality. Both are graded using exactly the same GIA/IGI criteria.
The "better" method depends entirely on what you're looking for.
Choose HPHT if: You value a stone grown in a single, continuous process without post-growth colour treatment. HPHT diamonds that come with an "As Grown" certification have not been treated after growth — which some buyers find satisfying from a purist perspective.
Choose CVD if: You prioritise exceptional chemical purity and consistently high clarity. CVD diamonds are typically Type IIa — the purest diamond classification possible — and tend to produce very clean stones. The HPHT colour treatment that many CVD diamonds undergo is stable, permanent, and industry-standard.
What doesn't change between the two: the hardness, the sparkle, the fire, the durability, and the fact that you are wearing a genuine diamond with a fascinating story.
Trivia 💡: When you look at a CVD diamond's IGI certificate and see the phrase "This Laboratory Grown Diamond was created by Chemical Vapour Deposition (CVD) growth process and may include post-growth treatment" — that's not a warning. It's transparency. It's the certificate telling you exactly what was done, openly and honestly. Compare that to the mined diamond industry, where treatments to existing natural diamonds are sometimes disclosed inconsistently. Lab grown diamonds win on transparency, every time.
Can You Tell the Difference Between CVD and HPHT Diamonds by Eye?
No. Not even close.
The differences between CVD and HPHT diamonds — growth patterns, internal strain characteristics, trace inclusion types — require sophisticated spectroscopic analysis to detect. The GIA uses techniques including infrared spectroscopy, photoluminescence spectroscopy, and UV fluorescence to identify growth method. These are not tools available at a jewellery counter. They are laboratory instruments.
To any human eye, both CVD and HPHT diamonds look exactly like diamonds. Because they are.
What About Coloured Lab Grown Diamonds?
Both methods can produce coloured lab grown diamonds — and this is one area where laboratory precision genuinely opens possibilities that nature rarely delivers.
- Yellow and orange diamonds can be produced in the HPHT process by introducing nitrogen during growth
- Blue diamonds can be achieved by introducing boron — replicating the conditions that produce some of the rarest and most expensive natural diamonds on earth (including the legendary Hope Diamond, a natural Type IIb)
- Pink diamonds are typically produced via CVD, followed by HPHT treatment to achieve that coveted blush colour
- Green and other fancy colours can be achieved through a combination of growth conditions and post-growth irradiation treatment
The result: colours that would cost hundreds of thousands of dollars in a natural stone are accessible and affordable in lab grown form. Science, as it turns out, is quite the democratiser.
The Environmental Angle
One often-overlooked aspect of understanding these two processes is what they tell us about the environmental footprint of lab grown diamonds.
HPHT requires significantly more energy to maintain those extreme pressures. CVD, operating at lower pressures, is generally considered the more energy-efficient of the two methods. Both processes, however, share the critical advantage of requiring zero mining — no land excavation, no water table disruption, no displacement of ecosystems.
We'll explore the full environmental picture — including an honest look at the energy consumption of lab growing — in Post 7: The Environmental Case for Lab Grown Diamonds.
Frequently Asked Questions
Q: Is a CVD or HPHT diamond better quality?
A: Neither is universally superior. Both can produce exceptional gem-quality diamonds. CVD diamonds tend to achieve higher purity classifications (Type IIa), while HPHT diamonds can be grown without post-treatment. Quality is assessed stone-by-stone using the 4Cs — not by growth method.
Q: Does the growth method affect a diamond's sparkle?
A: No. Brilliance, fire, and scintillation are determined by cut, not by how the diamond was grown. A well-cut CVD diamond and a well-cut HPHT diamond will sparkle identically.
Q: Are CVD diamonds weaker than HPHT diamonds?
A: No. Both score a perfect 10 on the Mohs hardness scale. The growth method does not affect hardness or durability.
Q: Will the growth method show on a grading certificate?
A: Yes. GIA and IGI grading reports for lab grown diamonds identify the growth method (CVD or HPHT) and note any post-growth treatments.
Q: Is HPHT treatment of a CVD diamond a problem?
A: Not at all. Approximately 75% of CVD diamonds undergo HPHT treatment to improve colour. It is a stable, permanent, and industry-standard process that does not affect quality or value.
Q: Can HPHT be used on natural diamonds?
A: Yes — HPHT treatment is used on some natural diamonds to improve their colour. This must be disclosed on grading reports, as it affects the stone's value classification.
Conclusion: Two Paths, One Destination
HPHT and CVD are the twin engines of the lab grown diamond revolution — different in their approach, united in their output.
HPHT channels the raw, brutal forces of the Earth's interior: pressure measured in tens of thousands of atmospheres, temperatures pushing toward the surface of a distant star, carbon dissolving and recrystallising around a seed in a choreography that mirrors geological time, compressed into weeks.
CVD constructs diamonds from the ground up, atom by atom, from a plasma cloud of methane and hydrogen, through a process that has no natural precedent — an entirely human invention that produces some of the purest diamonds ever analysed.
Both are extraordinary achievements of human ingenuity. Both produce real, certified, beautiful diamonds. And both are available in H&H Jewellery's lab grown collection, graded and certified to the highest international standards.
The only question left is: which one is going on your finger?
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