Okay, I'm writing a big arsed piece of exposition for the beginning of Rise of the Ring
, detailing what the Groupmind was up to for the last millenium and a half, and I'm a bit stuck on a section of worldbuilding (literally).
After the GM finally
get the last person Processed, it gets to work mining the Solar System for the raw material to break down into component atoms and recreate as the ill-defined Unobtanium to build the main structure of the Ring. Now I've got two things to go on to determine the mass
of this stuff. One was a mention of Quisling's Ringmetal collar, which Khan described to her as being both lighter than air and sufficiently tough that it would be simpler to cut her head off than cut through the material it's made of to remove it. The other is a throwaway gag in a drabble about the GM strip mining the Solar System for raw materials, including blowing up Pluto.
Okay, the lighter than air thing points towards Ringmetal being some form of Polymer Aerogel
, maybe reinforced by
carbon nanotubes. So, say whatever this stuff is made out of, it's got the density of maybe Helium. Now I have to figure out just how much
this stuff is going to weigh when it's used to build a structure over 260,000 km in diameter, 1,000 km wide and 10km deep, bearing in mind there's going be dense and heavy machinery, soil, water, and air in the structure as well. 
Now the mass of the Asteroid belt, even including Ceres and the rest of the Big Four, is still less than half of Charon, Pluto's moon. Now given the total guesstimated mass of the Ring, would that be enough to build it, or would the GM have to sacrifice some of the crappier smaller moons of Jupiter and Saturn to fill things in?
 I'm guesstimating the soil and rocks will go down to a depth of one, maybe two kilometers, and perhaps five for the deepest part of the Ring Seas.