The problem the press solves
Diamond is a form of carbon that is stable only at very high pressure. In the deep Earth, carbon can crystallize as diamond because it is squeezed by the weight of rock above and heated by the surrounding mantle. A high pressure high temperature press, or HPHT press, recreates the key conditions in a small chamber. The pressure needed is on the order of several gigapascals, tens of thousands of times atmospheric pressure, and the temperature is typically well above one thousand degrees Celsius. Exact values depend on the process and the metal used, so treat any single figure as typical rather than fixed.
What happens inside the cell
The press does not grow diamond directly. It squeezes a small capsule called a cell, usually a few centimeters across or less, that holds the ingredients. Typically these include a carbon source such as graphite, a metal catalyst or solvent often based on iron, nickel or cobalt, and a diamond seed or seeds in growth setups. At pressure and temperature, the molten metal dissolves carbon from the source. Because diamond is the stable form under those conditions, carbon then precipitates out of the metal onto the seed as diamond. The cell also includes a heater, usually an electric resistance element, and a pressure-transmitting medium such as a ceramic or salt-like material that spreads the squeeze evenly and insulates the heat.
The way growth is set up, with carbon flowing across a temperature difference toward a cooler seed, is a standard approach. The prepared starting surface plays a large part, since seed crystals set the orientation and quality of what grows.
Why anvils matter
To hold such pressures, the press uses anvils made of very hard material, commonly cemented carbide, which is tungsten carbide in a metal binder. Force is applied by large hydraulic rams. The pressure on the sample can be much higher than the average pressure on the ram because the anvil faces are small relative to the ram area, acting like a lever for pressure. The anvils experience huge stress, so they have limited lives and are a major running cost. Gaskets of material such as pyrophyllite seal the cell and flow slightly under load, creating a supported edge that keeps the anvils from cracking.
The main press designs
Three designs are widely discussed in the literature.
Belt press. Two opposed anvils with tapered faces press into a ring-shaped belt that surrounds the cell. The belt gives lateral support and resists the sideways spread of the sample. This is among the earliest designs used for diamond synthesis and remains well known. Its pressure chamber is fairly roomy, and its simple geometry made it a workhorse.
Cubic press. Six anvils approach the cell from three perpendicular directions, squeezing a cube-shaped assembly. Cubic presses are widely used in industrial production because they are compact, can be run at high rates, and tend to apply pressure fairly evenly. The cell volume is usually smaller than in other designs, which suits producing large numbers of small crystals such as industrial diamond grit.
Split-sphere press. A multi-anvil design in which a set of steel segments forms a sphere around an inner set of anvils. The structure is built to reach higher pressures and larger cells, but is more complex and costly to build and operate.
Each design is a compromise between cell size, achievable pressure, cycle speed and equipment cost. Smaller cells in cubic presses yield many small crystals; larger volume designs can aim at bigger single stones, though larger crystals typically need longer runs and tighter control.
Controlling the run
Operators ramp pressure and temperature to chosen targets, hold them for a time, then lower both carefully. During the hold, temperature stability is crucial: even small fluctuations can produce defects or inclusions of metal in the crystal. The growth rate is typically slow for high quality crystals, often taking many hours to days for larger gem-quality stones, while rapid growth is used for smaller, lower quality industrial grit. Crystal growth conditions strongly influence color, clarity and shape.
Nitrogen from the air or from the materials commonly enters the crystal and gives a yellow tint. Getters, metal additives that bind nitrogen, are sometimes used to reduce it. Intentionally adding boron makes blue, electrically conductive diamond. Both nitrogen impurities and boron doping change color and electrical behavior.
What presses are used for
HPHT growth is a common route to industrial diamond grit used in saws, grinding wheels and drill bits. It is also used to sinter polycrystalline diamond, where diamond powder is bonded under pressure with a metal binder to form cutters. In those cases the press does not grow single crystals but welds powder into a solid body, a step that is related to the polycrystalline diamond cutters used in drilling. HPHT is also used to alter the color of existing diamonds.
Limits and trade-offs
Compared with vapor growth, HPHT cells are small, which limits the size of single crystals, and metal inclusions are a recurring quality concern. Anvil replacement and the energy needed to power the press are significant costs. Vapor methods can produce large flat plates and coatings, which presses cannot do. The trade-offs are often summed up as HPHT versus CVD.