volcanic rock, known as "tuff."

I've seen people carving bricks to make houses with it in Nicaragua, using the same type of tool he is using. Tuff is easy to carve, but it is still a rock and doesn't dissolve in water. It is very high in silica. Although I'm sure that it will break down eventually, I could see it lasting several years at least.

The impressive thing to me is his precision of movement in how he "cuts" the rock. Again, I've seen others do it, but nothing as impressive as how well he is doing it.