In case you don't hear directly from the architect, I'd have to guess that it's Corten steel. It is a beautiful material, but you have to be careful with the details as it can stain and corrode adjacent materials. Corten develops a nice patina, the exact color of which can depend on the specific climate of the site. We've used it on several projects - here are a couple of links in case you're interested:

http://eggfarkarch.com/meadow-creek-house
http://eggfarkarch.com/boat-bay-house

The answer above is exactly right - thanks !