So after some playing around, as requested, here were some results. Though admittedly little science was done, it's a bit underwhelming I'm afraid.
It is indeed considered by the game a stone or boulder, and not a metal. It's just called faceted metal mind you, not a faceted metal bar. I just assumed when I first saw it, that it was a metal bar; but not so, as far as the game is concerned. It is to the game a stone, whose name is simply faceted metal. Confusing, I know!
I could only treat it as stone; it couldn't be smelted, nor could I use a metal forge to make a faceted metal item, for example a short sword. Doing so resulted in "need faceted metal bar". I could, however make stone stuff out of it. The most interesting thing I could think of was, of course a shortsword. It had listed as its uses "make sharp blades" so I figured that, like obsidian, I could make a stone sword.
Sure enough, that worked. I ended up with a faceted metal shortsword. It weighed less than 1 Urist, and appeared to be superior to steel, as there was very little in the way of deflects against steel armor (I sent him to attack a steel armor wearing gobby invader, as I had given steel making to goblins).
However, the metal in my game that is equivalent to vanilla adamantine (I basically changed adamantine's name to mithril, and that's all), appeared to be superior. I had no way of testing it against someone wearing mithril armor, so I loaded up the arena using that save's raws. Assuming the faceted metal listed in it was the same stuff I was messing with in the save, the mithril deflected it lots. So, all in all, an interesting occurrence, but it appears that the faceted metal in my game is somewhat inferior to vanilla adamantine, which is what I recall reading in a thread where some science had been done on the new metals.