The GHz Ultrasonic Interferometry is used to determine the sound wave velocities of minerals at high pressure in a diamond anvil cell. Because GHz Interferometry does not use any optical component, this method is applicable especially for opaque minerals such like magnetite, franklinite or magnesiowüstite.
To know more about GHz Interferometry see: Spetzler et al., Pure and Applied Geophysics, 141 (1993) Reichmann et al., American Mineralogist, 83 (1998)
Fig.: Diamond anvil cell (DAC) in front of an oscilloscope. Under the DAC the ultrasonic attachment with the ZnO transducer visible, which generates sound waves in the Gigahertz region.