The medium is a produce of the tree in the photo here. This kind of "tree" is abundant in our forest, so I guess it's renewable. But of course it may take quite some time for them to grow to size.
The logs then would be cut into "slates" (?) for mounting species orchids. The photo shows the back of the mount and you can see the orchid roots went through the slate in this photo.
Those that are not big enough for slates would be chopped into something like this and used as potting medium.
When I buy this stuff, I would normally cut it into smaller "grains" (?) so that the roots of the plants can easily penetrate and attach themselves.
This stuff - chopped tree fern, or Dớn in Vietnamese - is very porous, in fact it holds no water except what it has absorbed.
My above mentioned plants love this medium. They grow and bloom for me regularly, even non-stop like my anthurium.
PS: This post is for Lynn.