HPHT Growth

HPHT means high pressure, high temperature. In the context of this chapter, the term denotes a method for synthesizing a new diamond crystal, not the subsequent HPHT treatment of an existing diamond.

The method uses conditions under which diamond is thermodynamically favored, but the industrial process is not simply a miniature version of Earth’s mantle. Cell geometry, the metal solvent, a controlled temperature gradient, the seed, and growth rate are technologically engineered elements.

Historical Development

General Electric publicly announced successful HPHT synthesis of diamond in 1955. Much of modern single-crystal growth is based on the temperature-gradient approach developed in the late 1950s.

Early commercial production consisted primarily of industrial material. Only after decades of advances in controlling nitrogen, temperature, press stability, and growth did the more routine production of larger gem-quality crystals become possible.

Representative Process Conditions

For a general description of HPHT growth, GIA gives temperatures of approximately 1300–1600 °C and pressures above approximately 870,000 psi, or on the order of several gigapascals.

These are not universal recipes. Actual pressure, temperature, time, flux composition, and gradient depend on press design, target size, color, chemical purity, and the producer.

A process range is not an identification constant of the stone.

The Press and High-Pressure Cell

The industry has used several architectures:

• belt press;
• cubic press;
• BARS/split-sphere designs;
• other high-pressure configurations, including toroidal systems in certain technical contexts.

For gemological understanding, the important point is not the name of the press but that the cell must maintain stable pressure and a controlled temperature distribution around the carbon source, flux, and seed.

[VISUAL 49.1: Schematic HPHT cell—carbon source, metal solvent-catalyst, temperature gradient, and seed]

Carbon Source and Metal Solvent-Catalyst

In typical gem-quality HPHT growth, a source of highly purified carbon dissolves in a molten metal system. Common technological families include Fe, Ni, Co, and their alloys, with various additives.

The term catalyst is historically widespread, but solvent-catalyst is functionally more precise because the molten metal transports dissolved carbon from the hotter region toward the seed, where diamond crystallizes.

Metals used in the process may leave traces or inclusions, but their absence from a polished stone is not proof that the stone is not HPHT-grown.

Temperature Gradient and Seed

Growth is controlled by the temperature difference between the carbon source and the seed. In the hotter region, carbon dissolves more readily in the molten metal. As it is transported toward the slightly cooler seed, supersaturation and deposition of carbon in the diamond lattice occur.

The seed determines the initial crystallographic orientation. The new crystal is not “a piece of graphite glued on,” but a crystalline continuation on a diamond substrate.

Nucleation and Single-Crystal Growth

For a gem-quality product, the objective is controlled growth of a single crystal without uncontrolled nucleation of a large number of new crystallites. Temperature, pressure, and chemistry must remain stable long enough for the crystal to grow without excessive formation of defects, fractures, or metallic inclusions.

Faster growth may increase productivity, but it is not automatically better. Quality depends on the entire process window.

Growth Sectors

An HPHT crystal often develops multiple crystallographic sectors, including {100} and {111}, while other sectors may also be present depending on growth conditions.

Different sectors may incorporate different amounts of:

• nitrogen;
• boron;
• metallic impurities;
• other defects.

A single rough crystal may consequently exhibit internal zoning in color, luminescence, electrical conductivity, or spectroscopic characteristics.

[VISUAL 49.2: HPHT crystal with {100} and {111} sectors—different incorporation of nitrogen, boron, and metal-related defects]

Atomic Types and As-Grown Color

Many HPHT crystals rich in isolated nitrogen may be type Ib and yellow to orange-yellow. The process can be optimized for material with very little nitrogen and for near-colorless/type IIa products. Boron can produce type IIb material and, at higher concentrations, blue color.

An important modern nuance is that type IIb does not automatically mean a blue face-up appearance. In GIA’s submission population of HPHT-grown D–Z diamonds examined since 2020, more than 80% contained infrared-detectable uncompensated boron and therefore belonged to the type IIb category. This figure describes stones submitted to GIA, not global production, but it clearly shows that traces of boron are no longer limited to visibly blue HPHT products.

Other colors are also possible, whether from as-grown defects or after additional processing. The rule “HPHT = yellow type Ib” or “type IIb = blue HPHT” is therefore unacceptable as a final identification.

Growth Remnants and Metallic Inclusions

HPHT-grown diamonds may contain:

• metallic flux remnants;
• graphite;
• carbide or nitride phases;
• microscopic cavities and other manufacturing defects.

A metallic inclusion can be a very useful indicator within an appropriate body of data. Chapter 43, however, showed that natural diamonds may also contain metallic Fe–Ni–C–S inclusions. “Metal = HPHT” is therefore an erroneous rule.

Magnetism and Electrical Conductivity

Metallic remnants may give some HPHT diamonds a measurable magnetic response. Boron-doped type IIb material may be electrically conductive.

Both phenomena depend on the particular chemistry and quantity of defects. The absence of magnetism does not rule out HPHT, and conductivity is not exclusive to HPHT because natural type IIb diamonds exist.

Strain and Fractures

HPHT growth does not guarantee a perfectly strain-free state. Differences among sectors, inclusions, growth rate, and cooling can create local strain. Fractures may form during growth or later, while the rough is extracted and processed.

The complete atlas of strain and growth defects remains in Chapter 51.

Multiple Crystals in One Cycle

Modern HPHT systems can grow multiple crystals simultaneously in the same press. This does not turn the process into serial production of atomically identical stones. The position of each seed within the cell, the local gradient, and chemistry can still affect the result.

Producer size records become outdated quickly and are unnecessary for understanding the method; the web edition should keep them outside the stable core of the chapter.

HPHT Growth Versus HPHT Treatment

This is one of the most important terminological distinctions in the entire book.

HPHT growth creates a new diamond crystal.

HPHT treatment acts on a preexisting diamond—natural or laboratory-grown—to alter defects and often color.

The same physical concepts of high pressure and temperature may be involved in both processes, but their purpose and the stone’s status are entirely different.

[VISUAL 49.3: HPHT growth versus HPHT treatment—“creating a crystal” and “subsequently modifying an existing crystal”]

Why One Sign Is Not Enough for Identification

None of the following signs is a universal verdict by itself:

• yellow color;
• type Ib;
• metallic inclusion;
• magnetism;
• sectoral fluorescence;
• relatively low strain;
• a particular external rough morphology.

Reliable identification combines multiple gemological, spectroscopic, and luminescence data. That is the subject of Chapter 53.

Process Sequence

A professionally simplified HPHT workflow looks like this:

1. select the target product properties;
2. prepare the seed;
3. prepare the carbon source and metal system;
4. assemble the high-pressure cell;
5. increase the pressure;
6. establish the operating temperature and gradient;
7. maintain controlled growth;
8. complete the cycle with controlled cooling;
9. decompress the system;
10. open the cell and extract the rough;
11. document growth morphology and quality;
12. plan any processing and cutting.

Chapter Summary

• HPHT growth is a method for growing a new diamond, not a synonym for HPHT treatment.
• The method uses pressures on the order of several GPa and temperatures of approximately 1300–1600 °C in typical commercial descriptions.
• The actual recipe depends on the equipment, chemistry, and target product.
• Belt, cubic, and BARS presses are different high-pressure press architectures.
• Carbon is typically transported through a metal solvent-catalyst.
• A temperature gradient directs crystallization toward the seed.
• An HPHT crystal may contain multiple growth sectors.
• Different sectors may incorporate nitrogen, boron, and metallic impurities differently.
• HPHT-grown diamonds are not all yellow or all type Ib.
• Modern D–Z HPHT products may be type IIb with traces of boron but without an obviously blue face-up appearance; GIA’s figure of >80% applies to its submission population since 2020, not to global production.
• A metallic inclusion or magnetism may be indicative but is not a stand-alone test of origin.
• Natural diamonds may also contain metallic phases.
• Final identification of HPHT origin requires a combined body of laboratory evidence.

[VISUAL 49.4: HPHT workflow—press cell → carbon dissolution → transport through flux → crystallization on seed → rough → polished]