Great comic as always. I think there is an error in the last panel, I think it should be "in that case" instead of "in the case".

Thanks for letting me know! ^_^

Is this the first appearance of the kairoseki?

I think the first appearance is Smoker's Beating Stick, but that's the Canon universe.

I think it's been mentioned before now in the comic universe, IIRC it was referenced when 'luffy' got caught in the same shackles as Gol D. Roger

Kairouseki is weird. Diamond hard metal that can also be made into flexible ropes or whatever else is convenient?
But then again, it seems to be Phlebotinum so *shrugs*

I mean, are steel fiber cables really so implausible?

If you look closely at seastone nets, it's ess that the nets themselves are made of seastone and more that they have seastone spheres where the ropes cross

Granted, without seastone, the one piece world would most likely quickly become a dog-eat-dog hell in which whatever semblance of the Marines would have no choice but to regularly slaughter devil fruit users because they would be unable to restrain them.