Characterization of the Ge concentration in a Si1-xGex heterojunction for x varying from 5% to 40% in steps of 5% is done by bevelling and wet thermal oxidation of the exposed layers. The resulting difference in oxide thickness as a function of Ge concentration is visualized due to light interference. Different Ge concentrations are seen as different colours through an optical microscope. The technique—CABOOM—stands for characterization of alloy concentration by bevelling, oxidation and optical microscopy. The oxidation temperature and time to visualize 5% differences in Ge concentration has been optimized for CABOOM. We established that the bevelling method and smoothness do not have a significant influence on the results. A mathematical model was developed in Mathematica™ to study the limits and strengths of the technique. The results of the calculations are in excellent agreement with the measurements. Mathematical calculations are used to study the limit in layer thickness and Ge concentration resolution.