The colour of Ruby is largely related to electronic excitations occurring within the 3d-shell of Cr3+ impurities in the Ruby’s Al2O3 matrix. X-ray absorption spectroscopy at the Cr-L2,3 edge is a very direct experimental, element-selective probe of these 3d states and has been extensively employed over the last decades to understand the electronic properties of ruby. Unfortunately, due to the complexity of this material involving strongly correlated 3d electrons, the modelling of these spectroscopic signatures has mostly relied on semi-empirical approaches so far. The recent development of very efficient numerical schemes for dealing with strongly correlated electronic systems offers, however, new opportunities to gain insight into the physics of ruby. The objective of this master thesis is to use state-of-the-art techniques to calculate XAS signatures in ruby entirely from first-principles based on density functional theory, Wannier functions and constrained Random Phase Approximation.