HTC has had aluminum phones for a while, and I still believe that even though their design has gotten a little boring lately, the phones are still the most solid on the market and have been for the last couple of years. The solid aluminum backing ensured the phone's strength and resistance when falling on the ground, but it also gives a premium feel to the device.
But HTC is not sitting still, and since releasing the first such phones, they've kept working on new methods to improve the manufacturing process of their casing, to make it even stronger and of higher quality. And it seems they've achieved this through a new process that is called micro arc oxidation, that puts their regular aluminum through 10,000 volts until the backing becomes a very strong ceramic material.
The advantages are that it becomes 5x stronger than the old aluminum and 3x stronger than stainless-steel. That's pretty impressive, and it's even more impressive when you find out that it's used in space for satellites. I have to admit this is making me interested in HTC phones again if it's coupled with the S4 processor (which does happen in One S), and it might be good news if HTC actually lands the next Nexus device.
For a look into how they are doing this, see the video below: