Neil, when I was at Contano, Saint Gobain were developing a flexible and unbreakable glass.
We had a piece in the office to play with, they were using it for street light lenses to be vandel proof, we were unable to break it, even had it in the factory trying our best.
Apparently they fill in the microscopic holes (glass is a series of holes!), it then has flexibilty and is as strong as steel, but with added weight advantages, but very expensive.