It was nice to see an Apple materials scientist explaining it. I really wish that team could find a way to move away from glass backs though. The weight difference between the iPhone 4/4s and the iPhone 5/5s was really noticeable and I think a lot of that was the move away from the glass back to aluminium. Of course now Apple has wireless charging to contend with which really restricts the material choices for the back, and Apple would insist on whatever is used looking very premium, but I would think you need to be pretty much at the top of your game to land a job as a material scientist at a company of Apple's prominence and Apple has big R&D budgets so I'm not giving up hope that maybe one day we might get a phone that doesn't use glass for the back.
Not that complex. Hard wood.