It was Fender who did the granite guitar.
As for nut improvements, there have been plenty - delrin, graphite, floyd rose, the LSR roller nut, earvana...you get the idea. Glass would improve sustain, but it's very brittle, and while it would probably work for a while, I think the vibration of the strings coupled with the tension it carries would eventually break it over time.
Also, if you dropped the guitar or knocked into anything with the neck, chances are it would shatter.