text: The study of materials at extreme conditions, high pressure and high temperature uses a wide array of techniques to achieve these conditions and probe the behavior of material while in the extreme environment. Percy Williams Bridgman, the great pioneer of high-pressure research during the first half of the 20th century, revolutionized the field of high pressures with his development of an opposed anvil device with small flat areas that were pressed one against the other with a lever-arm. The anvils were made of tungsten carbide (WC). This device could achieve pressure of a few gigapascals, and was used in electrical resistance and compressibility measurements. 
  The first diamond anvil cell was created in 1957-1958. The principles of the DAC are similar to the Bridgman anvils, but in order to achieve the highest possible pressures without breaking the anvils, they were made of the hardest known material: a single crystal diamond. The first prototypes were limited in their pressure range and there was not a reliable way to calibrate the pressure.
  The diamond anvil cell became the most versatile pressure generating device that has a single characteristic that sets it apart from the other pressure devices – its optical transparency. This provided the early high pressure pioneers with the ability to directly observe the properties of a material while under pressure. With just the use of an optical microscope, phase boundaries, color changes and recrystallization could be seen immediately, while x-ray diffraction or spectroscopy required time to expose and develop photographic film. The potential for the diamond anvil cell was realized by Alvin Van Valkenburg while he was preparing a sample for IR spectroscopy and was checking the alignment of the diamond faces.
  The diamond cell was created at the National Bureau of Standards (NBS) by Charles E. Weir, Ellis R. Lippincott, and Elmer N. Bunting. Within the group, each member focused on different applications of the diamond cell. Van Valkenburg focused on making visual observations, Weir on XRD, Lippincott on IR Spectroscopy. The group members were well experienced in each of their techniques before they began outside collaboration with university researchers such as William A. Bassett and Taro Takahashi at the University of Rochester.
  During the first experiments using diamond anvils, the sample was placed on the flat tip of the diamond (the culet) and pressed between the diamond faces. As the diamond faces were pushed closer together, the sample would be pressed and extrude out from the center. Using a microscope to view the sample, it could be seen that a smooth pressure gradient existed across the sample with the outermost portions of the sample acting as a kind of gasket. The sample was not evenly distributed across the diamond culet but localized in the center due to the "cupping" of the diamond at higher pressures. This cupping phenomenon is the elastic stretching of the edges of the diamond culet, commonly referred to as the "shoulder height". Many diamonds were broken during the first stages of producing a new cell or any time an experiment is pushed to higher pressure. The NBS group was in a unique position where almost endless supplies of diamonds were available to them. Customs officials occasionally confiscated diamonds from people attempting to smuggle them into the country. Disposing of such valuable confiscated materials could be problematic given rules and regulations. One solution was simply to make such materials available to people at other government agencies if they could make a convincing case for their use. This became an unrivaled resource as other teams at the University of Chicago, Harvard University, and General Electric entered the high pressure field.
  During the following decades DACs have been successively refined, the most important innovations being the use of gaskets and the ruby pressure calibration. The DAC evolved to be the most powerful lab device for generating static high pressure. The range of static pressure attainable today extends to 640 GPa, much higher than the estimated pressures at the Earth's center (~360 GPa).