HPHT and CVD are the two methods used to grow diamonds. Both produce real diamond from a diamond seed. They differ in the conditions they use, in the shape of the crystals they grow and in the clues they leave for gemmologists.
At a glance
| HPHT | CVD | |
|---|---|---|
| Principle | Carbon crystallises from a molten metal solution under extreme pressure and heat | Carbon from a gas plasma deposits on seed plates, layer by layer |
| Pressure | Very high, about 5–6 GPa | Below atmospheric pressure |
| Crystal shape | Often cuboctahedral | Tabular, built in layers |
| Typical inclusions | Metallic flux inclusions | Dark pinpoints of non-diamond carbon |
| After growth | Can be cut directly; some stones are treated | Often treated to improve colour |
| Identification | By laboratory instruments | By laboratory instruments |
Does it matter which one you choose?
In practice, much less than the grade of the finished stone. A well-cut CVD diamond and a well-cut HPHT diamond of the same grade will look the same to the eye. What you should look for is the same as for any diamond:
- the quality of the cut;
- its colour and clarity;
- an independent report that states laboratory origin and, where applicable, any post-growth treatment.
About post-growth treatment
Many CVD diamonds grow with a brownish tint and are treated with heat and pressure afterwards to improve their colour. This is common practice and does not make the stone less of a diamond, but it should be disclosed. Ask whether a stone has been treated, and check its report.
Diamond types, briefly
Gemmologists classify diamonds into types according to the nitrogen and boron they contain. Most natural diamonds are type Ia, containing aggregated nitrogen. CVD diamonds are usually type IIa, with very little nitrogen, a type that is rare among natural diamonds. Colourless HPHT diamonds are also often type IIa, and some contain traces of boron. These distinctions matter to laboratories identifying a stone; for the wearer, they have no effect on beauty or durability.
Colour and fancy colours
Both methods can produce colourless and near-colourless stones. HPHT growth can also produce blue diamonds when boron is present, and yellow ones with nitrogen. CVD crystals often grow with a brownish tint that is removed by treatment. Pink and other fancy colours may result from treatments after growth. Every treatment should be disclosed.
What the report tells you
- That the diamond is laboratory-grown, stated clearly.
- In many cases, the growth method, CVD or HPHT.
- Whether post-growth treatment was detected, where the laboratory reports it.
- The usual 4Cs, measurements, polish, symmetry and fluorescence.
A buying checklist
- Judge the stone by its grades and its appearance, not by its growth method.
- Ask whether it has been treated after growth.
- Check the report number with the laboratory.
- Compare similar stones graded by the same laboratory.
Sources: Gemological Institute of America (gia.edu), resources on laboratory-grown diamonds.
Frequently asked questions
Is CVD or HPHT better?
Neither method is better in itself. Both can produce excellent diamonds; judge the finished stone by its grade and report.
Does the report say which method was used?
Reports always state that a diamond is laboratory-grown; some also indicate the growth method or post-growth treatment.
Are CVD diamonds real diamonds?
Yes. CVD is simply a method of growing diamond; the result is diamond.
Do HPHT diamonds contain metal?
Some contain tiny metallic inclusions from the growth process. In a well-graded stone they are usually invisible to the eye.
