Yeah, the stuff definitely is a ceramic (read: it consists of molecules made of metals that have been oxidized into being chemically rather inert, with those molecules arranged in a rigid crystal lattice through medium-strength EM interaction - an ionic crystal). It's very brittle, but like most ceramics, it takes compression stresses well - so it is a good choice for the outer layer of armored glass. In order not to shatter, it needs to be backed up by a flexible polymer inner layer, though. And Wikipedia informs us that it's also backed up by a layer of ordinary amorphic glass in the armor application.
AlON is apparently more crystalline than yer average quartz (that is, the crystalline order stretches across greater areas - but neither material consists of a single giant crystal, at least not with current manufacturing techniques), which helps against shattering. Both AlON and quartz are superior in this respect to ordinary glass, which isn't crystalline at all (glass may actually consist of the same molecules as quartz, yet any internal order only stretches across a couple of molecules). Due to lacking that crystal order, ordinary glass is less brittle, but also less capable of resisting compression.
Still, all these materials become armored glass the same way: by being the compression-absorbing layer, backed up by a flexible layer. AlON can do its job with a single pair of layers, while ordinary glass and its polymer backing need to be stacked dozens of pairs high in order to stop bullets. That means more weight and poorer transparency...
A diamond would be even better, because it is organized in a rigid lattice - but not in a lattice of molecules. Instead, all the atoms there latch individually on to their neighbors with very strong EM forces, so the entire diamond is actually a single molecule of carbon (in the ideal case). Something like that would be very difficult to achieve with a substance that contained appreciable amounts of aluminum or any other metal: the metal would want to chemically react with specific types of fellow substances, and the atoms would no longer individually latch on to their companions, but would want to do that ionic-crystal thing with the weaker forces. And if the stuff were mostly aluminum, it would want to be a metal, and metals have the defining quality of not letting visual range EM radiation through - the way the metal atoms hold on to each other, through sharing their electrons in a great group orgy, dictates that the EM radiation is always absorbed.
Yeah, AlON is a substance that could perhaps be called "transparent aluminum". But it's better called "transparent alumina" or "transparent alumina&aluminum nitride" because the aluminum there exists in those chemical forms, bonded to other atom types. Depends on who's speaking and listening, though. The company marketing the stuff (or the Star Trek entity manufacturing those starship windows) is definitely not obligated to speak the exact jargon of 20th-21st century chemists!
Frankly, there is no reason to believe that the windows were weaker than the hull.
Indeed, there may be no such reason even in face of the ST:GEN evidence. The window broke, yes - but it appears that the hull broke as well! At least we see lots of broken wall when Picard rummages through his possessions.
STXI is more explicit about this sort of thing: we see the bridge ceiling, wall and window/monitor surfaces all break in equal cracks when the ship fights the black hole.
Timo Saloniemi
