High energy water jets are established in processing brittle, inhomogeneous materials like rocks and concrete. Despite their wide field of application, the failure mechanisms of these materials, especially the influence of inclusions, are not well known. This work examines the influence of grain inclusions on the fracture behaviour of a multiphase brittle material exposed to high energy water jet processing. The behaviour of the specimens is detected by mass removal measurements, microscopical observations and the mercury penetration technique. It is found that the failure is based on microcrack growth due to hydrostatic pressure. The fracture mechanical behaviour of the reference material changes considerably with the addition of aggregates. The addition of grains leads to a reduction of the threshold tool energy for the start of mass removal. On the other hand, the presence of inclusions permits a more reduced and controlled removal progress. The interfaces between matrix and grains are the preferred locations for crack growth and also for crack branching. The inclusions act as crack arresters and crack branchers. In the case of cracks growing through the grains, a higher amount of fracture energy is absorbed and the fracture performance is weakened.