This study deals with low-power (3.2 kW) microwave irradiation of a range of rocks and provide an indication of the suitability of certain rock types for this treatment method for rock fragmentation and mineral comminution. The influence of microwave irradiation was assessed physically by measuring rock properties and observing cracking phenomena and chemically by analysing the mineralogical and chemical composition before and after microwave irradiation. The results show that the reaction to microwave irradiation strongly depends on the mineralogy or the water content of the rock. Rocks with low content of microwave absorbers (like granite and sandstone) heat slowly and do not show strong physical changes, whereas rocks with better microwave absorption (basalt, mafic volcanites) heat quickly leading to significant physical changes. It is apparent that the presence of water either in the porous volume of the rock or in the mineralogical structure (e.g. clay minerals) adds to the destructive work with the help of the generated steam pressure.