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

If you're researching lab-grown diamonds for an engagement ring, you'll encounter the acronyms CVD and HPHT within the first few minutes. They refer to the two methods used to grow gem-quality diamonds in a laboratory, and most descriptions of them either over-simplify or over-complicate the distinction. Here is an accurate, accessible explanation — and a direct answer to whether you should care about which one you buy.

The basic chemistry of both methods

All diamonds — natural or lab-grown — are pure crystallised carbon. The challenge in growing a diamond is persuading carbon atoms to adopt the diamond crystal structure (cubic sp3 bonding) rather than the graphite structure (hexagonal sp2 bonding) that carbon defaults to at room temperature and pressure.

Nature does this through geological time and extreme pressure deep in the earth's mantle. Labs do it faster, through two different routes.

CVD: Chemical Vapour Deposition

CVD works at relatively low pressure (around 100–300 torr) but at elevated temperature (700–1,200°C). The process:

1. A thin diamond seed plate (a slice of existing diamond) is placed in a sealed reactor chamber.
2. The chamber is filled with a mixture of hydrogen and methane gas (or another carbon-bearing gas).
3. The gas mixture is ionised by microwave energy, creating a plasma.
4. In the plasma environment, hydrogen strips away non-diamond carbon forms, while carbon radicals precipitate onto the seed plate in the diamond crystal orientation.
5. Layer by layer, the diamond grows — approximately 1mm per week for gem-quality growth.

The result is a roughly cuboidal rough diamond crystal, which is then cut and polished by the same artisans who cut natural rough (and to the same proportions standards).

What makes CVD diamonds identifiable under lab conditions: The layered growth process can leave "graining" — alternating transparent and slightly off-colour layers that are visible under cross-polarised microscope light. Modern reactors run tighter controls and produce much cleaner stones than early 2015–2019 CVD production. A good modern CVD stone shows little or no graining.

HPHT: High Pressure High Temperature

HPHT mimics natural diamond formation conditions more directly:

1. A carbon source (graphite or diamond powder) is placed around a diamond seed in a growth cell.
2. A metal catalyst/flux (typically nickel-iron alloy) surrounds the carbon source.
3. The cell is placed in a large hydraulic press and subjected to approximately 5–6 GPa of pressure (roughly equivalent to the pressure 150km below the earth's surface) and 1,300–1,600°C of heat.
4. The metal flux melts and dissolves the carbon source; the dissolved carbon then recrystallises around the diamond seed in the diamond crystal structure.
5. The process completes in hours to days (much faster than CVD).

HPHT diamonds are identifiable under lab conditions by characteristic growth sector boundaries — angular zones within the crystal that grew at slightly different rates. These zones can show subtle colour differences under crossed polarisers.

Side-by-side comparison

Feature	CVD	HPHT
Pressure required	Low (~200 torr)	Very high (5–6 GPa)
Temperature	700–1,200°C	1,300–1,600°C
Growth rate	Slow (days to weeks)	Faster (hours to days)
Crystal shape	Cuboidal/tabular	Octahedral (like natural)
ID marker	Layered growth striations	Growth sector boundaries
Common issues (historical)	Greyish haze in early stones	Metallic inclusions from flux
Market share (2025)	~70%	~30%
Typical applications	Gem-quality engagement stones	Some gems; also industrial

Does the production method matter when buying?

Our honest answer: less than most articles suggest. Both methods produce stones that range from poor to exceptional in visual quality. Both are identifiable only with laboratory equipment. Both are graded by the same labs using the same criteria. Both are priced similarly at retail.

What matters far more than production method:

1. The certificate grade — specifically cut quality, which drives more visual performance than colour or clarity in most engagement ring sizes.
2. The actual visual quality of the specific stone — which is why we look at every stone we source, lab-grown or natural, before offering it to a customer.
3. The reputation of the grading lab — we use IGI-certified stones for lab-grown; we check that the certificate matches the stone.

If you're buying a lab-grown diamond for an engagement ring, ask for the IGI or GIA report, check that it states the production method, and then let the 4Cs and the appearance of the stone be your guide — not the production route.

We're happy to show you examples of both CVD and HPHT lab-grown stones at our Murray Place boutique and explain what you're looking at. Call 01786 462799 to book a diamond viewing.