Apple had specially designed the new 7000 Series aluminium for the Apple Watch, which is 60% stronger than regular aluminium, and has one-third the density of stainless steel. In addition, the Series 7000 aluminium is machined, polished and blasted with microscopic zirconia beads to give them a consistent, satin texture on the Apple Watch Sport. The outer surface also goes through an anodising process that helps to prevent scratches and corrosion.
Apple currently uses anodised aluminium on the iPhone 6 and iPhone 6 Plus. While they look and feel good, they easily pickup dents and scratches during regular use. The use of Series 7000 aluminium will hopefully prevent this from happening on the upcoming iPhones.