Towards analyzing the breakage behavior of materials with respect to their physical properties in the context of simulation and modeling of the process of comminution in a circular fluid energy mill, the present work was taken up to characterize the effect of material hardness on the breakage process of the mill. The breakage parameters of a range of materials, chosen based on their hardness, were analyzed by estimating them using the G-H solution for the size discretized batch grinding equation, with corrections to account for the deviation from first-order kinetics during the initial stages of grinding and a back-calculation scheme. All the operating variables of the mill were kept the same while collecting the primary breakage data using a single-size feed for six successive passes through the grinding chamber of the mill. The breakage distribution values could be related to particle size with the sum of two power function equations, while the breakage rate could be expressed as a power function of the dimensionless particle size and the top size interval disappearance. The coefficients of these functions for different materials indicated inconsistent behavior with respect to the hardness for softer materials, while medium and harder minerals showed specific trends.