This is the most written-about question in lab-grown diamonds and the least important for a buyer. GIA's own explainer covers the chemistry in detail 1; this page gives only what you need to decide, and then points you to the pages that matter more.

Both are diamond

HPHT (high pressure, high temperature) grows diamond from a seed in a metal flux under conditions that mimic the Earth's mantle. CVD (chemical vapour deposition) grows it layer by layer from carbon-rich gas in a vacuum chamber 1. The product of each is crystalline carbon with the same hardness, refractive index and dispersion as a mined diamond 5. Nothing about the method changes how the stone looks, wears or lasts. A jeweller cannot tell them apart by eye, and neither can you.

The two differences that occasionally show

HPHT: a faint blue nuance. Boron in the growth chamber can leave HPHT stones with a very slight blue tint, which GIA's review of HPHT synthetics describes among their identifying characteristics 2. In colourless grades this is rarely visible in a set stone, but it is worth asking about if you are buying a large stone in D to F colour and want it dead white. Some buyers like it; a cold, icy white is a look.

CVD: brown or grey, and treatment. CVD crystals often grow with a brownish or greyish cast. The industry's answer is post-growth HPHT annealing, which removes it; GIA's research summary says that since 2020 about 80 per cent of CVD stones submitted to it show signs of post-growth processing 3. The treatment is permanent. The buyer-relevant point is disclosure: IGI reports since 2021 state either "as grown" or "may include post-growth treatment" 4, and an untreated CVD stone that has come out clean is the simplest thing to own. The certificate page shows where to find that line.

What to do with this

  • Do not pay a premium for either method. There is no quality hierarchy between HPHT and CVD; a retailer who charges more for one is charging for a story.
  • Read the report, not the brochure. Growth method and treatment status are both on an IGI report.
  • Look at the stone in daylight, next to a known white reference if you can. A tint you cannot see does not exist for practical purposes.
  • Spend your attention where it moves the result: the cut grade, the colour and clarity you actually need, and the setting. The colour and clarity page has the practical advice; the price page has the numbers.

That is all a buyer needs on HPHT versus CVD. The rest of this record is about the things that are less well covered.