As a result of its singular strength and exceptional transparency, diamond plays a unique role in laboratory studies of materials under extreme pressures and temperatures. Historically, the quality and size of the diamond used have been limited to that available from nature. However, researchers can now synthesize various types of diamonds in the laboratory that are harder, tougher, and larger than those available natural diamonds. The physical properties and features of these diamonds allow scientists to study materials at pressure-temperature conditions ranging from those at Earth's surface to those at the very center of the Earth. In essence, these synthetic diamonds offer new windows into the deep Earth.