The material that forms the skeletons of the higher animals. It consists of an organic portion composed mainly of the fibrous protein collagen and an inorganic portion formed by crystals of hydroxyapatite, a complex of tricalcium and calcium hydroxide. The combination of these two substances forms a material that is light and strong, especially longitudinally. It is fairly flexible and possessed of reasonable cross-sectional strength. It is readily available as a by-product of butchery and is easily worked with simple tools. It can be sawn, scraped, carved, filed and glued. The size and structure of even the largest bones tend to limit their utility to small objects. These artefacts tend to be utilitarian in nature, though they may be decorated to some extent, and bone was often used to adorn objects made from other materials. Bone has sometimes been used as a cheap substitute for ivory, but it cannot be so finely worked, and the grain is more prominent. For tools it was often the preferred material because of its greater strength in small sections.