Dragonbone

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 A Song of Ice and Fire Dragonbone is consistently described as being black in color, which the hilt of Petyr Baelish's Valaryian steel dagger reflects. Despite this, the dragon skulls stored in the dungeons beneath King's Landing are shown to be white.