Introduction
High Pressure, High Temperature (HPHT) synthesis was the first method used to produce gem-quality diamonds in a laboratory. The principle is conceptually straightforward: recreate the conditions that form natural diamonds deep in the Earth — extreme pressure and extreme temperature — and allow carbon to crystallise on a diamond seed in a controlled environment.
The process was originally developed for industrial diamond production in the 1950s. Gem-quality HPHT diamonds became commercially viable later, and today the method produces stones exceeding 10 carats in gem quality. China is the dominant producer, followed by Russia and India.
This article explains the growth process, the equipment involved, the conditions required, and the characteristics that distinguish HPHT-grown diamonds from natural stones and from diamonds grown by the alternative CVD method.
The Growth Process
Starting Materials
An HPHT growth cell contains three essential components:
- A carbon source — typically high-purity graphite
- A metal flux catalyst — an alloy of iron, nickel, and cobalt (Fe-Ni-Co) that acts as a solvent for carbon at high temperatures
- A diamond seed crystal — a small, pre-existing diamond on which the new crystal will grow
The seed is positioned at the cooler end of the growth cell, the carbon source at the hotter end, and the metal flux fills the space between them. When the cell reaches operating conditions, the flux melts, dissolving carbon from the graphite source. Because the seed sits at a slightly lower temperature, carbon precipitates out of the flux solution and deposits on the seed — atom by atom, building the diamond crystal.
Conditions
- Pressure: 5–6 gigapascals (GPa) — roughly 50,000 to 60,000 times atmospheric pressure. This is comparable to conditions at 150–200 km depth in the Earth's mantle.
- Temperature: 1,300–1,600 °C — sufficient to melt the metal flux and keep carbon in solution.
- Growth time: Hours to several weeks, depending on the desired crystal size and quality. Larger, higher-quality crystals require slower, more carefully controlled growth.
Press Designs
Three mechanical press designs are used in commercial HPHT diamond production:
Belt press. The original design, developed by General Electric in the 1950s. Two opposing anvils compress the growth cell, contained within a belt of pre-stressed steel rings. Reliable and well-understood, but limited in the sustained pressures and volumes it can achieve for large gem-quality stones.
Cubic press. Six anvils arranged along three axes compress the growth cell from all directions simultaneously. This geometry achieves more uniform pressure distribution than the belt press and can produce larger crystals. Widely used in Chinese production facilities.
BARS press (split-sphere). Developed in Russia, this design uses a split-sphere anvil system to generate pressure. It is efficient, relatively compact, and capable of producing high-quality gem crystals. The acronym stands for the Russian abbreviation of "Barometric Apparatus of Russian Scientists."
Crystal Growth and Morphology
Natural diamonds grow primarily as octahedral crystals — the classic eight-faced shape determined by the cubic crystal system under natural mantle conditions. HPHT-grown diamonds develop a different habit: cuboctahedral, combining cube and octahedron faces. This difference in growth morphology creates distinct internal growth sectors visible under specialised imaging.
The growth sectors matter because they affect how trace elements are incorporated. Nitrogen, for example, may concentrate in certain sectors and not others, producing colour zoning that is absent or different in natural diamonds. Under UV fluorescence imaging tools like the DiamondView, these sector patterns are among the most reliable indicators of HPHT origin.
Characteristic Features
Metallic Flux Inclusions
The most distinctive feature of HPHT-grown diamonds is the presence of metallic flux inclusions — tiny particles of the Fe-Ni-Co alloy trapped within the crystal during growth. These inclusions appear dark and opaque under magnification and can sometimes make the diamond weakly magnetic — a property virtually never found in natural diamonds.
Not all HPHT diamonds contain visible metallic inclusions. Higher-quality growth produces cleaner stones, but the potential for metal traces remains inherent to the flux method.
Colour Variations
The colour of an as-grown HPHT diamond depends primarily on the atmospheric composition within the growth cell:
- Yellow: Nitrogen from air enters the crystal lattice. This was the most common colour in early HPHT production. The nitrogen creates a Type Ib diamond (isolated nitrogen atoms), which is extremely rare in nature.
- Colourless: Achieved by controlling the growth environment to exclude nitrogen. Requires a nitrogen-free atmosphere and careful process management.
- Blue: Produced by introducing boron into the growth environment, creating a Type IIb diamond.
Post-growth treatments can further modify colour. Irradiation followed by annealing can produce pink, green, and other fancy colours from HPHT-grown starting material.
No Natural Mineral Inclusions
Natural diamonds often contain mineral inclusions — garnet, olivine, pyroxene — trapped during formation in the mantle. These inclusions are geological fingerprints. HPHT-grown diamonds never contain them. The absence of natural mineral inclusions, combined with the presence of metallic flux, is a strong indicator of laboratory origin.
Scale and Geography
China dominates HPHT diamond production, accounting for the majority of global output. The technology has matured to the point where gem-quality HPHT diamonds exceeding 10 carats are no longer unusual. Russia (notably New Diamond Technology) and India are also significant producers.
Production costs have fallen as press technology has improved and manufacturing has scaled. This cost reduction is one of the factors driving the overall decline in lab-grown diamond prices.
Frequently Asked Questions
How long does it take to grow an HPHT diamond?
Growth time ranges from hours to several weeks, depending on the size and quality of the desired crystal. Larger, gem-quality stones require slower, more carefully controlled growth.
Can HPHT diamonds be colourless?
Yes. By excluding nitrogen from the growth environment, producers create colourless HPHT diamonds. This requires careful atmospheric control, as nitrogen is the most common impurity in diamond growth.
How can you tell an HPHT diamond from a natural one?
Metallic flux inclusions, cuboctahedral growth sector patterns (visible under DiamondView imaging), and specific spectroscopic signatures distinguish HPHT diamonds from natural stones. Some HPHT diamonds are also weakly magnetic due to trapped metal flux. See Microscopy Indicators and Fluorescence Imaging.
What is the largest HPHT diamond ever produced?
Gem-quality HPHT diamonds exceeding 10 carats have been produced, with the largest reported stones approaching 15–20 carats. The size ceiling continues to rise as press technology improves.
Summary
HPHT diamond synthesis recreates the Earth's mantle conditions in a mechanical press — dissolving carbon in a molten metal flux at 5–6 GPa and 1,300–1,600 °C, then allowing it to crystallise on a diamond seed. The process produces genuine diamonds with characteristic features: cuboctahedral growth sectors, potential metallic flux inclusions, and colour determined by atmospheric composition. Three press designs (belt, cubic, and BARS) serve commercial production, with China as the dominant manufacturer. The method is mature, scalable, and capable of producing gem-quality stones that rival the best natural diamonds in appearance — distinguishable only by the internal signatures of their laboratory origin.