Obsidian is the name of the first book of the Mystic Stones Series and is also the only thing that can kill vampires in the Mystic Stones world. The characters use daggers and throwing knives carved from obsidian to kill vampires. The obsidian must pierce the heart of a vampire to kill it.
Obsidian is a volcanic glass that contains a large amount of silicone. Lava with high silicone content form obsidian when it cools rapidly. When broken, obsidian forms an extremely sharp edge such as the edge a shard of glass would have, therefore, if a vampire were encased in obsidian, it would be very dangerous for that vampire to try to break out.