The main one is decomposed granite. I will have to look at the quarries in the area and see what they have. Decomposed granite is used a lot for landscaping, as the city is moving to a "less greener" look to conserve water.
I am moving a few trees hopefully this weekend, I will take some pictures. I guess it depends on the bag, some bags don't entrap the roots, just prevent them from pushing through and eventually they start circling the bag just like a pot. The Rootpouch brand entraps the root tip, pruning it and creating finer ramification. If you look at cmeg1
CM Bonsai thread, I knew about grow bags for a while, but never thought about using them for bonsai till I saw his thread. That link is of one of his JBP seedlings, this is what I expect from my trees as well.
There is another brand that claims to do the same thing as Rootpouch. Root maker.
RootMakers® are designed to create fibrous, non-circling root systems for easy transplantation.
rootmaker.com
There is only one distributor for Rootpouch in Canada, Hydrofarm Canada.
https://hydrofarm.ca/pots-containers?specs=4309
We manufacture fabric planting containers, made from recycled water bottles and natural fibers. Designed in partnership with professional growers across the world.
rootpouch.com