A long time ago in a galaxy not very far away, Fred (zAmboni) mentioned transparent aluminum which is actually alumina (aluminum oxide). Now there's another transparent aluminum offering being tested by the Air Force: aluminum oxynitride. It's being tested as a replacement for the multi-layered glass windows that are being used now. The substance is more durable than glass, and it weighs less. In fact, its performance is nothing short of impressive:
In a June 2004 demonstration, an ALONtm test pieces held up to both a .30 caliber Russian M-44 sniper rifle and a .50 caliber Browning Sniper Rifle with armor piercing bullets. While the bullets pierced the glass samples, the armor withstood the impact with no penetration.
In extensive testing, ALONtm has performed well against multiple hits of .30 caliber armor piercing rounds -- typical of anti-aircraft fire, Lieutenant La Monica said. Tests focusing on multiple hits from .50 caliber rounds and improvised explosive devices are in the works.
And before you ask the question I know all of you Star Trek fans have on your mind, I'll give you the answer: 6 inches (at least according to posts on that site).