As unbreakable as ... glass?
To truly appreciate how glass can be used structurally, make your way to 233 South Wacker Drive in downtown Chicago. More precisely, make your way 1,353 feet above South Wacker, to the 103rd floor of the Sears Tower.
Once there, take a few steps over to the west wall, where the facade has been cut away. Then take one more step, over the edge.