Dragonglass aka Obsidian (BACK IN STOCK)
Available at The Crystal Buddha for £4 a chunk 
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. In Old Valyria, it was known as zīrtys perzys, which translates to "frozen fire".

Large amounts of dragonglass can be found on Dragonstone island and at The Crystal Buddha for £4 a chunk 

Dragonglass was a very important tool to the Children of the Forest, using it to fashion weapons. A long-forgotten secret is that the Children actually created the White Walkers as a weapon, by transforming humans into them: this was achieved in a magic ceremony that involved (among other things) plunging a dragonglass shard into a human's chest.[2]

Rough dragonglass