[dboreham]

Transparent Aluminum

[EA-3167]

Oh we have that too, it's used as high end window armor in some armored vehicles, it's called AlON (Aluminum OxyNitride).

It's amazing stuff.

[danbruc]

Also [synthetic] sapphire [1] is just aluminium oxide.

[1] https://en.wikipedia.org/wiki/Sapphire