Don't forget neutrons! Not as quick to measure, and not so good with very small samples, but well-suited to combinations of extreme environments beyond just pressure, such as temperature, magnetic field, voltage gradient etc. https://www.isis.stfc.ac.uk/Pages/Pearl.aspx
Amusing to see for an x-ray crystallographer that the neutron scattering coefficient for tungsten carbide is actually lower than for pure carbon. Neutrons are weird.
Contents are completely enclosed by diamond to hold the pressure. No way to probe otherwise as far as i know.