Corrected entry: Based on references from Star Trek IV and TNG it has been made clear that the windows are made of transparent aluminium and not of glass, but when the saucer section crashes, some windows shatter like normal glass.
harry8472
22nd Jul 2013
Correction: Having wrecked my motorcycle in an accident, I can tell you that aluminum definitely shatters with enough force. There is a video on the Internet showing what happens when a .50 caliber bullet hits transparent aluminum (yes, it does exist) and at the impact point of the shot, you can see fragments that look very similar to shattering glass flying away it.
rswarrior