Diamonds provide our only direct samples of the very deep Earth and understanding their origin is consequently of great interest to Earth Scientists. The origin of diamonds can be investigated via the grains of other minerals included within them as they grow; these inclusions also provide unique information about deep Earth mineral assemblages. An inclusion in a diamond may be under a remnant pressure, from which the source pressure of the diamond may be inferred. Coesite inclusions are ideal for this purpose as, unlike olivines or garnets, their chemical composition is fixed and so the properties of the inclusion at atmospheric pressure are known. However, coesite inclusions commonly give diamond source pressures that are demonstrably too low. This application addresses a possible source of this error by determining the thermal expansion of coesite to 1750 K.