[citation][nom]Travis Beane[/nom]How about aluminium oxide? I think sapphires are pretty hard. Synthetic sapphire glass is hard, but brittle. Scratch resistant, but it can still shatter. I've succeeded in scratching gorilla glass, but hey, it's never shattered after numerous drops.[/citation]
The brittle aspect is the major kicker as aluminum oxide are ceramics that need to be thicker, special additives that can affect the viewing quality, and/or just too damn expensive for the application. AlO_ may be overkill in this situation (remember this is transparent armor kind of sh!t).