The stability of structures constructed in or on rock mass depend on rock mass deformability. Deformability of rock mass shows significant spatial variation due to structural features such as joints, bedding planes, foliations etc. However, variation may or may not be significant depending upon the behaviour of discontinuities. Variability in deformability of rock-mass with space results from many known and unknown sources of uncertainties. Inherent variability can be modelled as a random field in terms of Mean and coefficient of variation (COV) of modulus. Based on in-situ experimental study of deformability of Granite, Sandstone, and Phyllite rockmass from three different geologies in India, the COV of deformability are evaluated along with the mean values for which the COVs are applicable. To measure the variation in values of modulus a simple procedure is proposed in the paper. Furthermore, based on trends observed for COV of modulus with scale, a model is proposed for COV in terms of scale of measurement. With the knowledge of mean and COV of deformation modulus at any project site, a randomfield type modelling can be done at the desired scale.