Titanium. An amazing 30 percent stronger than steel yet 50 percent lighter, titanium has gone mainstream in the industry. It's also highly corrosion-resistant making it an excellent choice for diver watches in particular. A downside: it's expensive
Ceramic. Temperature resistant, hypo-allergenic, and impervious to rust or scratches, ceramic is an ideal material for watches. It can be highly polished and is frequently used to create bezels.
DLC. DLC stands for diamond-like Carbon and carries many of the same properties of its expensive namesake: it's an extremely hard material and virtually impossible to scratch.
Composites. More and more watch companies are blending metals to create their own special composite watch cases. The Panerai Radiomir Composite Marina Militare case to the left is made of a combination of aluminum and ceramic. The result: a watch case harder than anything except DLC and lighter than all.