I wondered what you were talking about, so I google searched "Knapped old colored TV screens" and saw this:
Is TV tube glass worth THAT much trouble? I'd rather buy a nice thick piece of leaded glass crystal from a TS, GS, ES or whatever, if thickness and flatness was the key quality. I don't knap anything, so aside from being free, is tube glass special?
Hmm, I will say I have used a carbide tipped hammer to save some time on grinding glass. It works a lot like knapping.. If you do it enough you have a good idea which way the breaks will run. I have never sat down with the intention to knap anything.
Certain pieces of glass have a tenancy not to shatter and endure a lot of stress. Depends on how they were annealed and how thick they are. I don't think there is much special about it. Maybe its thick and chips well. I would assume a lot more thought went into annealing tv glass. So its less likely to explode in your hand.
I'm not into scraping but I would say when he brushes the dust off the inside of the monitor that probably is some sort of dust that you really don't want to breath on the inside of old tv's. If you want to smash one up I don't think you are going to fall over dead but I would not make a hobby of it without some respiration.
If you just want a large chunk of colored glass to knap I would look up Blenko Dalles.