A device for producing very high pressures. A sample of material to be subject to the high pressure is placed in a cavity between two high-quality diamonds. The diamond-anvil cell operates like a nutcracker, with pressures up to 1 megabar (1011 Pa) being exerted by turning a screw. The pressure exerted can be determined by spectroscopy for small samples of ruby in the material being compressed, while the sample itself is observed optically. A use of the diamond-anvil cell is to study the insulator-metal transition in such substances as iodine as the pressure is increased. This type of study is the nearest laboratory approach to the structure of matter in the conditions obtaining in the interior of the earth.