Dragonbone is a durable material that, as it's name suggests, is harvested from the bones of dragons. Dragonbone is highly prized among weaponsmiths, as it is as resilient as metal, yet lighter and more flexible.
In the books
In the A Song of Ice and Fire novels, dragonbone is black, 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.