HPHT vs CVD Lab-Grown Diamonds: What's the Difference?

What do HPHT and CVD mean for lab-grown diamonds?

HPHT stands for high pressure, high temperature. CVD stands for chemical vapour deposition. They are the two commercial methods used to grow diamond in a laboratory, and every lab-grown diamond on the Australian market comes from one or the other. Both make diamond: pure carbon locked in the same cubic crystal structure as a natural stone, with a hardness of 10 on the Mohs scale, a refractive index of 2.42 and a density of about 3.52 g/cm³.

That is why the answer to whether lab-grown diamonds are real diamonds is yes. The growth method changes how the crystal formed, and that can leave small clues inside it. It does not change what the material is. Our overview of how lab-grown diamonds are made follows the whole journey from seed to polished stone; this guide focuses on how the two methods compare once a diamond is in front of you.

A simple way to picture it: HPHT copies the conditions found roughly 150–200 kilometres below the Earth's surface, where natural diamonds form. CVD takes a different route nature never used, growing the crystal a few atoms at a time at low pressure. Different roads, same destination.

How is an HPHT lab-grown diamond made?

An HPHT grower places a small diamond seed crystal, a carbon source (usually high-purity graphite) and a metal solvent, typically an alloy of iron, nickel or cobalt, inside a growth cell. A press then applies about 5–6 gigapascals of pressure, roughly 50,000–60,000 times the air pressure around you, and heats the cell to about 1,300–1,600°C. The molten metal dissolves the carbon, which then crystallises onto the slightly cooler seed.

Most HPHT diamonds today come from cubic presses, which push on the cell from six sides at once; belt presses and split-sphere presses are also used. A growth run takes from several days to a few weeks, depending on the size of crystal the grower is aiming for.

What HPHT growth can leave behind:

• Crystal shape: a cubo-octahedron, with square cube faces and triangular octahedral faces. The cutter plans the polished stone around this shape.
• Metallic inclusions: tiny specks of the metal solvent can be trapped as the crystal grows. Under magnification they look opaque, shiny and dark, sometimes as grains, sometimes as rods, and larger ones can make a stone weakly magnetic. Our guide to metallic flux inclusions in HPHT stones shows what they look like.
• Blue nuance: growers add 'nitrogen getters' to keep nitrogen out and the stone colourless, and a trace of boron can slip in, giving a faint blue or grey-blue tint. See when a blue nuance in HPHT diamonds matters.
• Phosphorescence: many HPHT stones keep glowing blue-green for several seconds after shortwave ultraviolet light is switched off, a useful clue for a lab.
• Sector zoning: different growth sectors absorb impurities differently, which can show as a cross or hourglass pattern under UV imaging.

HPHT is very efficient for small stones, so a large share of lab-grown melee (the tiny diamonds in halos, pavé and eternity bands) is HPHT. Growers also produce colourless HPHT crystals large enough to cut stones of several carats.

How is a CVD lab-grown diamond made?

CVD starts with thin, flat seed plates of diamond, often sliced from earlier CVD crystals, laid out in a vacuum chamber. A gas mixture of hydrogen with a small proportion of methane is fed in, and microwave energy turns it into a glowing plasma well below normal air pressure, with the seeds held at about 700–1,200°C. Carbon separates from the methane and settles on the seeds, stacking up diamond layer by layer.

Growth is slow, typically a few to several tens of micrometres per hour, so a crystal thick enough for a 1–2 carat polished stone can take several weeks. Growers often stop, polish the top surface and restart to keep quality consistent.

What CVD growth can leave behind:

• Crystal shape: a flat, square-ish tabular block with a rim of non-diamond carbon that is cut away. Rounds, ovals and squares are all cut from it routinely.
• Dark pinpoints and clouds: specks of non-diamond carbon (graphite) rather than metal, so CVD diamonds are not magnetic.
• Growth striations: faint parallel bands from the layered growth, usually visible only under magnification or lab imaging. Read whether CVD growth striations affect the look before you rule a stone out.
• Brown or grey body colour: structural defects and traces of nitrogen can leave an as-grown tint, often reduced by treatment after growth. Our guide explains why some CVD stones show a brown or grey tint.
• Strain and fluorescence: CVD stones often show banded strain between crossed polarising filters, and some fluoresce orange-red under UV because of nitrogen-vacancy defects.

CVD scales well for larger, high-clarity stones, and it is the method behind much of the 1–3 carat lab-grown diamond in Australian jewellery stores.

HPHT vs CVD: what is the difference?

Side by side, the differences are about the process and the clues it leaves, not about how hard, bright or durable the finished diamond is.

Type IIa means a diamond contains almost no nitrogen. Only about 1–2% of natural diamonds are Type IIa, while nearly all colourless lab-grown diamonds of either method are. Type IIb contains a little boron, which gives a blue tint and lets the stone conduct electricity. Both facts become important when it comes to testing, further down.

Are CVD lab-grown diamonds treated after growth?

Often, yes. Many CVD crystals come out of the chamber with a brownish tint. Growers then anneal them: heat them, either in an HPHT press for a short time or at low pressure, without growing any new diamond. This reorganises the defects that cause the brown and can lift the colour several grades. It is a post-growth treatment, and it is stable under normal wear and care.

So 'HPHT' can appear on a CVD diamond's paperwork. In that case it describes the treatment, not the growth method. The same annealing is used on some mined stones, which is why natural diamonds improved by HPHT must be disclosed as treated, and why our guide to HPHT and irradiation colour treatments is useful if you are comparing coloured diamonds.

Some growers now produce CVD diamonds that need no treatment, and reports may describe these as 'as grown'. A treated stone is not worse by default. What matters is the colour and clarity you end up with, and that the report clearly says what was done. Coloured lab-grown diamonds such as pinks are often made with further steps, for example irradiation followed by annealing, and that should be disclosed too.

HPHT vs CVD: which looks better once it is set?

Neither, as a rule. Two diamonds with the same grades, one HPHT and one CVD, cannot be told apart without lab equipment. Differences appear stone by stone, not method by method:

• Cool tints: an HPHT stone graded D–F with a faint blue nuance can look very icy in white gold or platinum. In dull light a few people find it slightly steely.
• Brown or grey tints: a CVD stone at G–H can carry a hint of brown or grey instead of the usual yellow, which can look a little less lively than a warmer H.
• Inclusion type: clarity grades rate size, number and position of inclusions, not their origin. A VS2 is a VS2 either way, though a dark metallic speck under the table can draw the eye more than a pale pinpoint.
• Haziness: rarely, strong strain or striations in a CVD stone give a faintly hazy look that the grades don't fully capture. A video shows it better than a report.

Most lab-grown diamonds, including IGI-graded stones, are graded on the same D-to-Z colour letters as natural stones (GIA now uses a simpler descriptive system for lab-grown); how colour grades apply to lab-grown stones explains what the letters mean when the tint is blue, grey or brown rather than yellow. The biggest factor in how bright a diamond looks, whichever method grew it, is cut. Because lab-grown rough costs less, some stones are cut for weight rather than light, and our lab-grown cut quality guide shows what to check.

Can you tell HPHT from CVD with a diamond tester?

No. Standard thermal testers read both HPHT and CVD stones as diamond, because they are diamond. Our guide to how lab-grown diamonds react to testers goes through each type of device. One quirk: boron-bearing Type IIb HPHT stones conduct electricity, so some combined thermal-electrical testers can flag them as moissanite, a known false reading.

That confusion is a good reason to understand how moissanite differs from diamond and why moissanite can fool a basic tester. Cubic zirconia, by contrast, fails a thermal tester outright; see lab-grown diamond compared with cubic zirconia.

Telling HPHT from CVD takes lab tools used together:

Screening devices used by jewellers can flag a stone as laboratory-grown or refer it for further testing, but confirming the growth method reliably is a job for a gem lab such as GIA or IGI. For most buyers, the practical answer is simply to read it off the grading report.

HPHT vs CVD price: does the method change the cost in Australia?

Not by much. In Australia, lab-grown diamond prices follow carat, colour, clarity and cut far more than growth method. HPHT can be a little cheaper in small sizes and is common in very high colours; CVD is widespread in 1–3 carats. Lab-grown prices have fallen sharply over recent years and still move, so treat these as rough loose-stone ranges only.

What the report should say. A lab-grown report from IGI or GIA states that the stone is laboratory-grown, gives the growth method (HPHT or CVD) and notes any post-growth treatment. IGI reports, for example, carry comments such as 'As Grown – no indication of post-growth treatment' or a note that the stone was treated after growth. Many lab-grown stones are also laser-inscribed on the girdle with the report number and 'LG' or 'Laboratory-Grown'.

Australian rules. The Australian Consumer Law prohibits misleading or deceptive conduct, so a lab-grown diamond should be described as lab-grown or laboratory-grown on the label, the invoice and the report. If the growth method or a treatment note is missing from the paperwork, ask for it before you pay.

GemTrove sells both natural and lab-grown diamonds, so we have no reason to steer you towards one growth method. Natural diamonds are rarer; lab-grown diamonds cost much less for the same look. Both are real diamond.

Does the growth method change durability, care or insurance?

No. HPHT and CVD diamonds are equally hard and tough, scratch only against another diamond and clean the same way as a natural stone: warm water, a drop of detergent and a soft brush. Our guide on caring for a lab-grown diamond covers cleaning and ultrasonic machines. As with any diamond, tell your jeweller what the stone is before a repair that involves a torch.

For insurance, lab-grown diamonds are usually valued on replacement cost, which is far lower than for natural, so the insured value should reflect that. Keep the report and inscription number with your policy. Read more on insuring a lab-grown diamond, and look up unfamiliar terms in our jewellery glossary.

An HPHT or CVD diamond sets exactly like a natural one, in claws, a bezel, a halo or a band.

How to choose between HPHT and CVD: step by step

1. Start with shape, size and budget. The growth method is the last thing to decide, not the first.
2. Read the report. Confirm 'laboratory-grown', the growth method and any post-growth treatment note, and check the inscription number matches the report.
3. Ask about tint. Find out whether the stone has a blue, grey or brown nuance, then view it in daylight against white.
4. Look at the inclusions. On a video or under a loupe, check that no metallic speck or dark cloud sits under the table.
5. Judge the cut. Check proportions and light return; a well-cut stone of either method looks brighter than a poorly cut one.
6. Compare like with like. Put an HPHT and a CVD stone of the same grades side by side and choose the one that looks better to you.