Watch bezels have, until 2004 or so, been made from materials like aluminum, steel and plastics. Because of their prominent position, they often get damaged. Several companies have introduced tougher bezels to solve this. Seiko uses a sapphire coating on the GMT Grand Seiko, as does IWC on their new Aquatimer, and I think it was Rolex that introduced the use of ceramics, starting with rev-2 GMT.
Here's a pic from the Rolex page:
As of Oct 09, Omega has gone even further with their 'liquid metal' amalgam, which is primarily zirconium (also ceramic.)
Cool stuff, expensive but super durable. I've seen price quotes to replace a Rolex bezel for $1,500!