Dragonglass is a common name in Westeros for the substance known as obsidian, a form of volcanic glass. Along with Valyrian steel, it is one of the two known substances capable of killing White Walkers. It is the paramount tool-stone and major implement of the magic of the Children of the Forest, responsible for, amongst other things, transforming men into White Walkers, first performed during the Dawn Age. Large amounts of dragonglass can be found on Dragonstone.