I don't know the value of iron (or if your noble really likes iron), but you'd probably be better off smoothing and engraving the magnetite walls. Especially if you have a legendary engraver, I think you'd get more value out of that. Not that you need it, necessarily, but something to keep in mind for future reference. I should go see what the room values are for where I found all those emeralds...
And obsidian areas are ridiculously easy to make noble rooms out of. I tried making them in microcline, and it hasn't gone so well.
I don't know what my noble's likes and dislikes are either.
I considered just smoothing out the magnetite walls, but I went with this method because it was faster to mine out the whole thing and build for efficiency than it would have been to dig to the outside of the deposit, mine out the bordering stone, and work out how to build rooms in the magnetite. I think the first noble is going to show up in the next immigration wave, and I still need to build the consort's rooms. I'll probably just tack walls into a corner (I think the magnetite was surrounded by flux) and fix things as I go.
As a side note, two of the walls are partially built on non-magnetite stone. I don't know if it's more efficient to do it that way, but based on the responses here, I think it might be, although only slightly.
My confusion was based on:
24 tiles (floor) * 10 (material value of iron) * smoothing value (unknown) >= 2400. (Door plus bed or coffin = 100.)
That made smoothing's minimum modifier to value 10, which seemed excessive. Once I add in the walls, though, I have 48 tiles, which makes the smoothing value a minimum of 5, which seems a little more reasonable, as well as being in line with Jim Groovester's description.