From A Wiki of Ice and Fire
Dragonbone is the material that is created from the bones of dragons. It is used to make many items of value. This is because of its rarity and its special properties.
Properties of Dragonbone
Dragonbone is black. This is due to its high iron content. It is as strong as other metals, such as steel, yet lighter and more flexible. Dragonbone bows are highly sought after as they can fire arrows over a much farther range than normal wooden bows.