I came across these Seiryu stones on Facebook Market a couple of weeks ago. They seems to be similar to the Ibigawa stone that Ryan used for the Spruce of Stone creation a few weeks ago. Apparently they are used in a lot of aquarium scenery (I used to breed tropical fish years ago), but I had never heard of them.
These are acid treated, which I think Ryan mentioned was done to the Ibigawa stone as well.
Has anyone else seen or used these for tree on rock designs?