In crystallography, a diffraction standard, or calibration crystal, is a crystal used to calibrate an X-ray spectrometer to an absolute X-ray energy scale. A range of materials may be used including quartz or silicon crystals. There are also reports of crystals of silver behenate or silver stearate having been used for this purpose.
References
External links http://www.gwyndafevans.co.uk/thesis-html/node85.html http://www.ccp14.ac.uk/solution/calibration/index.html https://bigbro.biophys.cornell.edu/documents/SAX_Calibrants/saxs_sphere.html