Using a phase field model we explore crack propagation in bio-inspired composites in which the mineral and organic phases are arranged in a layered fashion. We show how the crack paths can be drastically altered by varying the elastic modulus mismatch between the organic and mineral layers, and by changing the thickness of the organic layer. Depending on the modulus mismatch and the thickness of the organic layer, the crack can either propagate straight, can branch inside organic layer or can get deflected along the interface, leading to delamination. The mechanism that governs the crack trajectories are analysed in terms of energy distribution near the crack tip. The critical energy release rate of the composite is also analysed as a function of the thickness of the organic layer and the modulus mismatch. A considerable enhancement is achieved when the ratio of the elastic modulus of the organic to mineral phase is less than 0.2. In such cases, for a given modulus mismatch, the critical energy release rate attains a maximum only for an optimal thickness of the organic phase. The origin of the optimal thickness is also investigated. © 2014 Elsevier B.V. All rights reserved.