Biomineral composites, consisting of intercrystalline organics and biogenic minerals, have evolved unique structural designs to fulfill mechanical and other biological functionalities. Aside from the intricate architectures at the composite level and 3D assemblies of the biomineral building blocks, the individual mineral blocks enclose intracrystalline structural features that contribute to the strengthening and toughening at the intrinsic material level. Therefore, the design strategies of biomineralized composites can be categorized into two structural levels, the individual building block level and the composite level, respectively. This dissertation aims at revealing the material design strategies at both levels for the bioinspired designs of advanced structural ceramics. At the building block level, there is a lack of comparative quantification of the mechanical properties between geological and biogenic minerals. Correspondingly, I first benchmark the mechanical property difference between biogenic and geological calcite through nanoindentation techniques. The selected biogenic calcite includes Atrina rigida prisms and Placuna placenta laths, corresponding to calcite {0001}, and {101 ̅8} planes. The natural cleavage plane {101 ̅4} of geological calcite was added to the comparative study. Under indentation load, geological calcite deforms plastically via twinning and slips under low loads, and shifts to cleavage fracture under high loads. In comparison, the P. placenta composites, composed of micro-sized single-crystal laths and extensive intercrystalline organic interfaces, exhibit better crack resistance. In contrast, the single-crystal A. rigida prisms show brittle fracture with no obvious plastic deformation. Secondly, how the internal microstructures and loading types affect the mechanical properties of individual building blocks is investigated. The prismatic building blocks are obtained from the bivalves A. rigida and Sinanodonta woodiana, where the former consists of single-crystal calcite and the latter consists of polycrystalline aragonite. The comparative investigation under different loading conditions is conducted through micro-bending and nanoindentation. The continuous mineral matrix in A. rigida prisms leads to comparable modulus under tensile and compressive loadings in the elastic regime, while the high-density intracrystalline nanoinclusions contribute to the conchoidal fracture behaviors (instead of brittle cleavage). In comparison, the interlocking grain boundaries in S. woodiana prisms correlate with easier tensile deformation (smaller tensile modulus) than compression, as well as the intergranular fracture morphologies. The third topic in the biomineral-level investigation focuses on how biomineral utilizes residual stress at the macroscopic scale. The selected model system is the spine from the sea urchin Heterocentrotus mamillatus, which has a bicontinuous porous structure and mesocrystalline texture. It is confirmed that the spine has a macroscopic stress field with residual tension in the central medulla and compression in the radiating layers. The multimodal characterizations on the spine conclude that the structural origins are not associated with the gradient distribution of the intracrystalline defects, including Mg substitution in the calcite matrix, intracrystalline organics, and amorphous calcium carbonates (ACC). It is hypothesized that the residual stress is generated due to the volume expansion during ACC crystallization at the compacted growth front. At the composite level, even though enhanced crack resistance is expected in biomineralized composites due to their hierarchical structures, the correlation between their 3D composite structures and damage/crack evolution is quite limited in the literature. I developed in-situ testing devices integrated with synchrotron-based X-ray tomography to capture the crack propagation in the materials, including the four-point bending and compression/indentation configurations. Two representative models are chosen to demonstrate the deformation of biomineralized composites under bending and compression, respectively, including the calcium carbonate-based gastropod shell (Melo diadema) and the hydroxyapatite-based fish teeth (Pogonias cromis). Also, the two composites are designed to achieve different functional requirements, i.e., enhanced fracture toughness vs. wear resistance. The comprehensive characterizations of these two composites revealed how biological structural composites are designed accordingly to their functional needs. For the crossed-lamellar M. diadema shell, directional dependence of the shell property was revealed, where the transversal direction (perpendicular to the growth line) represents both the stronger and tougher direction, but the longitudinal direction is more resistant to notches and defects. For the P. cromis teeth, the enhanced wear resistance of the near-surface enameloid originates from the intricate designs at the microscale, with c-axes of hydroxyapatite crystals and micro-sized enameloid rods coaligned with biting direction and F and Zn doping. In addition, the fracture morphologies of the fish teeth correlate with the microstructures; the enameloid exhibits corrugated fracture paths due to the interwoven fibrous building blocks, and the dentin exhibits clean planar fracture surfaces. / Doctor of Philosophy / Ceramic materials have wide applications in daily life and advanced technologies, and examples range from kitchenware (e.g., cups and plates) to spacecraft (e.g., thermal coating). These materials have indispensable applications due to their advantages of high strength and hardness, high heat and corrosion resistance, lightweight, chemical inertness, etc. Yet, intrinsic brittleness usually limits their applications. Typical ways to enhance the toughness of ceramics involve microstructure design (by refining the sizes and shapes of grains) and transformation toughening (phase transition) at the individual grain level, composite reinforcement (or ceramic matrix composites) at the composite level, and introducing residual stress to impede crack initiation and propagation. The engineering methods usually involve high energy input, chemical treatment, and usually significant waste and non-ecofriendly emissions. Therefore, learning the design strategies from biological ceramic solids constructed by organisms wound provide valuable insights into enhancing the performance of ceramics while reducing the harmful impact on the environment. In this dissertation, I investigated the mechanical design strategies from natural 3D biomineralized composites from two structural levels, i.e., building-block and composite levels, analogous to individual grains and composite reinforcement in engineering ceramics. For the building-block level research, the model systems include bivalve shells Atrina rigida, Placuna placenta, and Sinanodonta woodiana. The three bivalve shells contain different building blocks with intrinsic microstructures, corresponding to monolithic prisms with controlled nanoinclusions, diamond-shaped thin laths, and polycrystalline prisms with interlocking grains, respectively, presenting different structural designs of individual grains in ceramic materials. The sea urchin Heterocentrotus mamillatus spine represents a natural porous material with compressive residual stress on the surface, and the investigation of the structural origins aims to provide insights into the cost-effective synthesis of stressed ceramics with residual stress for engineering applications. In addition, the composite-level studies focus on the composite structures of the crossed-lamellar shell Melo diadema and the fish teeth from Pogonias cromis. These two model systems correspond to natural ceramic matrix composites with nano-scale fibrous building blocks arranged in 3D specialized for enhanced crack resistance and wear resistance, respectively. The comprehensive investigation of the deformation behaviors and mechanisms allows for a better understanding of the intricate strategies specialized for different functional requirements, which apply to bio-inspired designs in ceramic composites.