Nacre-like aluminas are bio-inspired, ceramic-based composites that display high toughness and strength. Their toughness arises from their brick-and-mortar microstructure. We must understand the role of several microstructural features over their mechanical properties. Techniques that allow microstructural imaging during mechanical tests are therefore desired. Here, we use both in situ fluorescence spectroscopy to image the stress field around cracks propagating in samples, and in situ scanning electron microscopy to image the crack-microstructure interactions. Stress concentrates around locally disordered zones where the crack is pinned while crack propagation is delayed. In situ imaging shows that obstacles to crack propagation are either larger-than-average alumina platelets or bundles of alumina platelets misaligned with respect to the majority of the platelets. Such microstructural heterogeneities are therefore important to impede crack propagation in nacre-like alumina. The approach proposed here can be used to understand the structure/properties relationships of other types of materials.