Bone is a composite material principally made up of a mineral phase (apatite) and collagen fibrils. The mineral component of bone occurs in the form of polycrystalline platelets 2–6 nm in thickness. These platelets are packed and probably glued together in stacks of two or more, ranging up to >30 platelets. Here we show that most of these stacks are curved flat sheets whose cylindrical axes are oriented parallel to the long axes of collagen fibrils. Consequently, the curvature of the platelets is not detectable in TEM sections cut parallel to the collagen fibril axes. The radius of curvature around these axes ranges from about 25 nm (the average radius of the collagen fibrils) to 100′s of nm. The shapes of these curved forms contribute to the compressive strength of bone.