This image shows the diamond anvil cell apparatus used by the Cornell Mineral Physics. In using the principle of Pressure=Force/Area, scientists squeeze materials to extremely high pressures via a very small sample area. Diamonds are well-suited as anvils to compress the sample to be studied -- they are the hardest material on Earth, and they are transparent to light. Thus lasers can be used to shine through the diamond anvils to heat the sample to temperatures of the Earth's deep interior. Studies using these type of experiments compliment seismic studies as we attempt to bridge the gap between the shapes and sizes of seismic velocity patterns (imaged from seismology) and their possible chemical composition.
Source: Cornell Mineral Physics