Bone is a truly incredible substance. It is as strong as steel but at as light as aluminum. Not only is it strong, but it is surprisingly flexible as well. As is the case with most things God made, human technology cannot come close to producing something with bone’s amazing properties.
Bones have several very strong minerals in them and I believe they take the form of a crystal lattice. That is also the shape of diamonds, so it's easy to see why they're so strong. (Only the osteoids)