Raw Tailstone Crystals
Here are some raw tailstone monocrystals. I haven't decided if tailstone can only be found naturally or if it can be synthesized. In either case, the best way to get tailstone currently involves going through the Partisans, since the Outer Belt has the highest concentration of tailstone (or tailstone components) in the system.
Maybe Earth, the moon, or other bodies at Sol are full to bursting with tailstone, which would have drastic implications for monkey fox geopolitics going forward, as the only reason the Partisans aren't a pariah state is they're a cheap source of tailstone, and a lot more besides, but tailstone is probably the yinrih's version of oil.
One detail I didn't work into the story above is that the giant monocrystals like you see in the image are fairly rare. Tailstone monocrystals are manufactured like silicon, where you have a small seed crystal that you get naturally that is grown in a bath to a single monocrystal. At some point in the formation process, the crystal structure “locks” and any wafers obtained from that crystal, or any smaller crystals broken from it, are interlinked via the underlay in the fashion mentioned in the story.
There are some big cybersecurity implications to this system. Say you ordered a batch of tailstone monocrystals from a supplier in Partisan terratory. The Partisan government could demand that the supplier grow larger monocrystals, and split them into several normal sized ones, offering half to the buyer and giving half to the Partisan government. Any communications made via underlay tunnels made with the half given to the buyer could be eavesdropped on by the Partisan government. There would be no way for the buyer to know the crystals were fractured before delivery, and as long as the Partisans don't send anything via their half, there's no way to know someone's listening.
There's also no way, physically, to verify the source of a message. All you know is the sender must have access to a wafer shaved from the same monocrystal you have, but there are ways of insuring nonrepudiation and message authentication that work as well for ansibles as they do on Earth. So yeah, lots of fun hacker shenanigans all around, especially when you put mass routers into the mix. You could poison the routing table and cause a whole freaking person to just show up somewhere they didn't want to go.
This concept probably won't be canon, but one idea I had was for “one way mass routers” that are used as oubliettes, just shunting people into the underlay with no way out.
Time will tell how long my ADHD brain will be interested in using Blender, but I'm already happier with these 3D models than I was with the 2D drawings. They're pretty simplistic, but it makes me nostalgic for the times when stuff with this little detail was considered top of the line graphics. Anyone remember the show Reboot? Loved that cartoon.