Okay, I'll concede the point on Mikasa due to my poor memory.
But regarding the rocks, unless proven otherwise I'll insist that there have been at best vague mentions of fuel or steam but nothing on rare ressources or even something so obviously useful. Of course, feel free to correct me if I'm again forgetting something.
As for the weapon.. I know it referred to the titans before but the new chapter seems more vague on it. But maybe that was just because the two topics followed each other so closely at least zeke sure made it sound as if there's more to the rocks than we know. But I guess I likely misread that part the first time.