The HPHT diamond process — how high pressure grows real diamonds

The GE breakthrough — how HPHT was born

The dream of making diamonds artificially is almost as old as the knowledge that diamonds are carbon. Once chemists understood that diamond and graphite were the same element in different crystal structures — confirmed by Smithson Tennant in 1797 — the question became obvious: could you force graphite to rearrange itself into diamond by applying enough pressure and heat?

The answer turned out to be yes — but achieving the necessary conditions took 150 years of materials science and engineering. The challenge was not just the conditions themselves (extreme as they are) but containing them. Building a pressure vessel that could withstand 870,000 psi without catastrophically failing — and still allow the experiment to be run safely and repeatedly — was an engineering problem of the first order.

GE assembled a team called Project Superpressure in the early 1950s. The team included physicist Francis Bundy, chemist Robert Wentorf, and engineer Herbert Strong — but the decisive contribution came from Tracy Hall, a Mormon scientist from Utah who had a talent for mechanical invention. Hall designed the "belt apparatus" — a press with two cone-shaped pistons compressing a cylindrical vessel that was itself enclosed in a steel belt to prevent explosive failure. The geometry allowed pressures and temperatures that could not be achieved in any prior design.

On December 16, 1954, Hall loaded his belt press with iron sulphide, graphite, and a tiny diamond seed and ran the experiment. When he opened the press and examined the carbon material, he found tiny octahedral crystals — the natural crystal form of diamond. He scratched them against everything he could find. They scratched everything. They were diamonds.

How HPHT works — the science

HPHT diamond growth is conceptually straightforward even though the engineering is immensely complex. The process has three components: a seed crystal, a carbon source, and a metal solvent catalyst.

The seed crystal is a small, carefully selected diamond — either natural or previously grown — placed at one end of the growth cell. Its crystal structure serves as a template for the new diamond to grow onto. The quality of the seed directly affects the quality of the grown crystal.

The carbon source is typically graphite or a form of processed carbon powder. This is the raw material that will be converted into diamond. The metal catalyst — usually an alloy of iron, nickel, and cobalt — is placed between the carbon source and the seed crystal.

When the press is activated and the temperature and pressure reach target conditions, the metal catalyst melts into a liquid. The carbon dissolves into this liquid at the hot end (where the graphite is) and supersaturates. Because the seed crystal is cooler than the carbon source, the dissolved carbon preferentially crystallises onto the seed — following its existing crystal structure and extending it, atom by atom, into a growing diamond crystal.

The process continues for two to four weeks. The press is then slowly cooled and depressurised, the growth cell is removed, and the rough diamond crystal — often a cubic or octahedral form depending on growth conditions — is extracted for assessment and cutting.

Cross-section of an HPHT growth cell — carbon dissolves at the hot end and crystallises onto the seed at the cooler end

The three HPHT press designs

Over the decades since Hall's belt apparatus, three distinct HPHT press designs have been developed, each with different capabilities, cost profiles, and growth characteristics. All three are in commercial use today.

The belt press

Hall's original design. Two opposing anvils compress a cylindrical capsule containing the growth cell. A steel belt around the capsule prevents it from expanding radially under pressure — this is the key innovation that allows much higher pressures than a simple opposed-anvil design. Belt presses are large, expensive to build and maintain, and have been primarily used by GE and De Beers' Element Six division for industrial diamond production. They are less common in Chinese commercial gem production.

The cubic press (or split-sphere press)

The cubic press uses six anvils approaching a central cube from all six faces simultaneously, applying isostatic pressure. This design became dominant in Chinese commercial HPHT production because it can be built more cheaply, maintained more easily, and scaled more readily than the belt press. A typical cubic press for gem production weighs several tonnes and occupies a significant factory floor area. Chinese manufacturers produce thousands of these presses — a single large HPHT diamond production facility in Henan Province may operate hundreds of them simultaneously.

The split-sphere press (BARS apparatus)

Developed by Russian scientists, the BARS (Barometric Apparatus with Reaction Sphere) press uses a spherical design that applies extremely uniform pressure. It is capable of producing very high-quality, large crystals. Russian producers — notably New Diamond Technology in St. Petersburg — have used BARS apparatus to produce some of the largest and highest quality HPHT gem diamonds ever grown, including stones above 10 carats of gem quality.

Machines and producers — who makes HPHT diamonds

The HPHT diamond production industry is dominated by Chinese manufacturers, with significant Russian and American producers serving the premium and speciality ends of the market.

What HPHT produces — quality characteristics

HPHT diamonds have specific quality characteristics that differ from both natural diamonds and CVD-grown diamonds. Understanding these helps buyers evaluate HPHT-grown gems.

Colour tendencies

Early HPHT diamonds were predominantly yellow or brown — caused by nitrogen contamination from the growth environment. Modern HPHT production can produce near-colourless stones by carefully controlling nitrogen content, but achieving high colour grades (D–F) consistently is more challenging in HPHT than in CVD. The majority of HPHT gem diamonds on the market are in the G–J colour range, with D–F stones commanding a premium.

Conversely, HPHT is excellent for producing fancy yellow and orange diamonds. By deliberately introducing nitrogen, producers can achieve vivid, saturated yellows that would take millions of years to form naturally. Fancy yellow HPHT diamonds are widely available and significantly cheaper than natural fancy yellows.

Inclusion types specific to HPHT

The most distinctive HPHT inclusion is metallic flux — tiny particles of the iron/nickel/cobalt catalyst that were not fully expelled from the growing crystal. These appear as dark, opaque inclusions under magnification. They are magnetic — an HPHT diamond with metallic flux inclusions can be attracted to a strong magnet, a test that no natural diamond would pass. The presence of metallic flux is a reliable indicator of HPHT growth.

HPHT diamonds also commonly show hourglass or cross-shaped graining patterns when viewed under cross-polarised light — a result of the cubic growth environment and the temperature gradient within the press.

Fluorescence patterns

HPHT diamonds typically show distinctive fluorescence patterns under UV light that differ from natural diamonds. Many HPHT stones show green, orange, or yellow fluorescence under shortwave UV — patterns that are unusual in natural diamonds. Under the De Beers DiamondView instrument (which uses shortwave UV), HPHT growth sectors glow in characteristic patterns that are diagnostic for lab-grown origin.

HPHT as a treatment — improving natural diamonds

Beyond growing new diamonds, HPHT is also used as a treatment process for natural diamonds — changing their colour to improve their commercial value. This is a separate and commercially significant application that buyers need to understand.

Many natural diamonds are brown — a colour caused by structural defects (plastic deformation) in the crystal lattice rather than chemical impurities. These brown diamonds are relatively common and less valuable than colourless or fancy-coloured equivalents. When subjected to HPHT treatment at appropriate temperatures and pressures, the structural defects can be annealed, removing the brown colour. The result is a near-colourless or fancy-coloured diamond that appears to be a higher-quality natural stone.

HPHT treatment must be disclosed on grading certificates. GIA and IGI both include a notation on the certificate when treatment has been detected — "HPHT annealed" or similar language. Undisclosed HPHT treatment is fraudulent. Detection requires the same spectroscopic methods used to identify HPHT-grown diamonds.

HPHT vs CVD — the key differences

HPHT in India

India's involvement in HPHT diamond production is primarily on the cutting and polishing side rather than the growing side. Chinese and Russian producers grow HPHT rough, which then flows through international trading channels to Surat for cutting and polishing — exactly the same route as natural rough diamonds.

Some Indian companies have begun investing in domestic HPHT growing capacity, particularly in Gujarat. The Indian government's 2023 budget decision to eliminate customs duty on lab-grown diamond seeds has lowered the entry cost for domestic production. However, the scale of Indian HPHT growing capacity remains small compared to Chinese production volumes.

The dominant Indian lab-grown story is in CVD, not HPHT. Surat's cutting factories, combined with growing CVD production capacity in Gujarat, have made India a significant CVD diamond producer and processor. HPHT diamonds processed in India are mostly imported rough from China.

Frequently asked questions

Is an HPHT diamond a real diamond?

Yes, completely. HPHT diamonds are carbon in a cubic crystal structure — chemically and physically identical to natural diamonds. The HPHT process replicates the conditions under which natural diamonds form; it does not produce an imitation or substitute. An HPHT diamond has the same hardness (10 Mohs), the same refractive index (2.417), and the same optical properties as any natural diamond.

Can HPHT diamonds be detected?

Yes, by spectroscopic methods — not by visual examination. Standard diamond testers cannot distinguish HPHT from natural diamonds. The De Beers DiamondView, GIA's iD100, and FTIR spectroscopy can identify HPHT-grown origin with high reliability. Metallic flux inclusions (where present) can also be detected by testing with a strong magnet. All certified HPHT diamonds will have "laboratory-grown" noted on their GIA or IGI certificate.

Why are some HPHT diamonds yellow?

The yellow colour in HPHT diamonds is caused by nitrogen atoms incorporated into the crystal lattice during growth. Nitrogen absorbs blue light and transmits yellow — the same mechanism that causes yellow colour in natural Type Ib diamonds. In HPHT growth, nitrogen from the growth environment and the carbon source can incorporate into the crystal unless actively excluded. Modern HPHT processes can control nitrogen levels to produce near-colourless stones, but producing consistent D–F colour HPHT diamonds remains challenging.

What is the difference between HPHT grown and HPHT treated?

HPHT grown means the diamond was created from scratch using the HPHT process — it is a lab-grown diamond. HPHT treated means a natural diamond that already existed was subjected to HPHT conditions to change its colour — it remains a natural diamond but has been processed. Both must be disclosed on grading certificates. Both are detectable by spectroscopy. The distinction matters commercially: an HPHT-treated natural diamond is a natural diamond that has been colour-enhanced; an HPHT-grown diamond is a lab-grown diamond. They are different products with different valuations.