As I have said here I believe that the dragons are as big thread against the humanity as the Others and that the world would be protected by balance against the extremities of fire and ice. Now I was thinking about obsidian. We know that obsidian is a volcanic glass made by cooled lava and that afawk is the only think that can kill the Others. For me I see obsidian as a balance between fire and ice, it’s lava but is cold and since it can kill one of the extremes it can kill the other