Gases? So vacuum is allowed?
I'll admit when I first read that detail in a description of the blimp-like airship the 2 characters are riding right at the beginning of the book, it took me a second to realize exactly what it meant. My understanding of airships like blimps and such hinges on something being in the gas bladder... But yes, nothing is certainly lighter than something! I haven't yet encountered any technical description of how the bladders are constructed, which I look forward to reading.
In a sci-fi book you would construct your unobtanium shell in space and use them to splash down heavier payload to earth. But the material needed is probably still magic.
I think Reynolds also had smelting in space, where molten metal would be turned into a huge volume of froth using some lighter than air gas. The 'burgs' of metal foam could then gently float down to earth.