Because of their transparency, thermal conductivity, or surface properties, diamonds are used in many research instruments as windows. An application of exceptional value in mineral and material science is a small device that generates extremely great pressures in the space between two diamonds - the diamond anvil cell. These devices are used in experiments on the nature of planetary interiors and dense matter, from mimicking Earth's core to producing solid hydrogen. The mechanics of creating high pressure are simple, involving just an application of force onto a small area, but extreme pressure will not be achieved without a material of supreme hardness, incompressibility, and strength - such as diamond. Most materials, steel for example, will deform or break before reaching pressures that exist deep within Earth.
Tungsten carbide is better, but diamond is best.
At these pressures hydrogen transforms into a metal - a state that might exist deep within Jupiter. Research on planetary interiors and dense matter has been advanced greatly by the use of diamond anvil cells, using lasers, optics, and x-rays to probe these small samples to reveal their mysteries. The diagram above illustrates the principle of the diamond anvil cell, and opposite it is a photomicrograph of an actual diamond anvil. To contain the pressurized area, a small donut-shaped metal gasket is placed between the diamond anvils.