A dedicated Geant4 study was developed to determine a correction factor (C) to convert the energy deposition response in diamond to water for heavy charged ions, with atomic number (Z) greater than 2 with energies typical of Galactic Cosmic Rays. The energy deposition response within an ideal diamond based microdosimeter was modelled and converted into a microdosimetric spectrum. The simulation was then repeated, substituting diamond with water. It was shown that by applying the correction factor, the energy deposition and microdosimetric response in diamond could be matched to that of water. The correction factor was determined to be C = 0.32 to 0.33. This study has shown a weak dependence of the correction factor C with respect to the Z of the projectile. The correction factor remains applicable for converting microdosimetric spectra in diamond to water for Galactic Cosmic Rays. This result is extremely encouraging and indicative of the applicability of diamond for use in radioprotection applications in space environments.