osage and sinew - both are magic materials (the king and the queen?)
As said by others, I would not back an osage bow when there is no good reason. Sinew adds lot of mass, so the bow should be in middle or better upper range of drawweight. Unbacked osage can handle lot of bend, esp. when build as bend through the handle bow you even can make very short bows. Here are some good examples on the forum.