That is actually a very beautiful volcanic stone that we imported from Bali, Indonesia just for this project. The stone was custom saw-cut for us oversees before shipping. It's a beautiful material with a warm gray color and dark flecks throughout. The stone is very soft so it is best for vertical surfaces where it won't receive a lot of wear. I have never seen this stone for sale in the US. The closest substitute you might find in the US is some kind of Andesite from South America.