Natural materials like bone, tooth, and nacre are nanocomposites of proteins and minerals with high strength. The nanometer scale is crucial for their strength and robustness. This study shows that the nanoscale of mineral particles in these materials is selected to optimize strength and tolerance to flaws. The traditional engineering concept of stress concentration at flaws is not valid for nanomaterials. Natural materials have hierarchical structures, with the smallest building blocks on the nanometer scale. For example, tooth enamel consists of needle-like crystals about 15–20 nm thick, while bone has mineral platelets around a few nanometers thick. Nacre has aragonite bricks around a few hundred nanometers thick. These materials have high fracture energy due to their composite structure, with mineral lamellae separated by soft layers and protein glue, which allows stress redistribution and crack stopping. The study shows that the high stiffness of biocomposites is achieved by the large aspect ratio of mineral platelets. A simple model estimates the stiffness of biocomposites based on the properties of mineral and protein components. The fracture strength of mineral platelets is optimized by their nanoscale size, which minimizes the effect of flaws. The critical length scale for this optimization is about 30 nm. When the mineral size exceeds this scale, the material becomes sensitive to flaws, but below this scale, it becomes insensitive. The study also shows that the optimal aspect ratio of mineral platelets is inversely proportional to their thickness. The mineral crystals in bone have a thickness of a few nanometers and an aspect ratio of 30–40, while those in nacre have a thickness of a few hundred nanometers and an aspect ratio of about 10. The optimal thickness of the protein layer between mineral platelets is also important for fracture resistance. The study concludes that materials become insensitive to flaws as soon as their structural size reaches this critical length. This result is significant for the design of new nanomaterials and highlights the importance of hierarchical structures in achieving complex mechanical properties. The study also notes that other factors, such as chemical environment and stoichiometric conditions, can influence the size of mineral crystals in biological systems.Natural materials like bone, tooth, and nacre are nanocomposites of proteins and minerals with high strength. The nanometer scale is crucial for their strength and robustness. This study shows that the nanoscale of mineral particles in these materials is selected to optimize strength and tolerance to flaws. The traditional engineering concept of stress concentration at flaws is not valid for nanomaterials. Natural materials have hierarchical structures, with the smallest building blocks on the nanometer scale. For example, tooth enamel consists of needle-like crystals about 15–20 nm thick, while bone has mineral platelets around a few nanometers thick. Nacre has aragonite bricks around a few hundred nanometers thick. These materials have high fracture energy due to their composite structure, with mineral lamellae separated by soft layers and protein glue, which allows stress redistribution and crack stopping. The study shows that the high stiffness of biocomposites is achieved by the large aspect ratio of mineral platelets. A simple model estimates the stiffness of biocomposites based on the properties of mineral and protein components. The fracture strength of mineral platelets is optimized by their nanoscale size, which minimizes the effect of flaws. The critical length scale for this optimization is about 30 nm. When the mineral size exceeds this scale, the material becomes sensitive to flaws, but below this scale, it becomes insensitive. The study also shows that the optimal aspect ratio of mineral platelets is inversely proportional to their thickness. The mineral crystals in bone have a thickness of a few nanometers and an aspect ratio of 30–40, while those in nacre have a thickness of a few hundred nanometers and an aspect ratio of about 10. The optimal thickness of the protein layer between mineral platelets is also important for fracture resistance. The study concludes that materials become insensitive to flaws as soon as their structural size reaches this critical length. This result is significant for the design of new nanomaterials and highlights the importance of hierarchical structures in achieving complex mechanical properties. The study also notes that other factors, such as chemical environment and stoichiometric conditions, can influence the size of mineral crystals in biological systems.